How can I know if my macbook is hacked? Today when I turned on my macbook I could see that the Desktop Background has been changed to Yosemite picture.
Then right after clicking on Change Desktop background... the background changed to the one I had, which was a green background with a dog in the center, except for the dog picture, it was only the green background.
After this I clicked again on Change Desktop background... and it changed again to the Yosemite picture.
Finally I went to finder file to see where the picture of the dog was, and when I clicked to see it, it was opened but at the same time 2 more pictures were opened.
After all this, I clicked again on Change Desktop background... but nothing happened. The Yosemite background didn't change this time.
All the time when this happened, my mac didn't have connection to internet.
Why? Is my mac hacked? Does it have a virus? What do I do?
Notes.

Every once in a while I see a message on the upper RHS (in the corner of the screen) telling me that I should update something related to the system but I always click on later and then the message vanishes.

Update
More details of the situation above.
3 pictures were opened by itself at the same time. I did not chose to open 2 of them, had I chose them then they weren't be displayed in the way they did because there is a standard way when one open several pictures at once. There 3 pictures displayed like  if I were selected 1, then the other and then the other within milliseconds.
That above plus the change of desktop background, it's 10000% strange.
I know that is stupid to think that I am hacked but have I another option than to this so?
So, how can I know if someone,  from another part in the world, is in my macbook?
Footnote. Related question of mine Why did my desktop background changed and why pictures were opened, if I did nothing to do so?
No.  You are not hacked.
Hackers do not operate in this manner.  There's this common misconception about hackers being in the dark recesses of their parents basement consuming copious amounts of Red Bull and Hot Pockets in an effort to brute force their way into your computer to change the background picture.
Hacking is 90% social engineering - it's exploiting vulnerabilities to get you to &quot;give up the goods&quot; like passwords, security questions, or any other sort of credential to gain access to a system.
So, what do they do when they gain access?
Try to find more credentials to gain access to other systems. Maybe they get a treasure trove of identity &quot;stuff&quot; (drivers license info, credit card or banking info, etc.) that they can sell online, but what they're looking for is more keys to get into other systems.  It's about a lateral move to the next system(s), not breaking the one they're currently on.
Changing your desktop picture and drawing attention to their activities would be a colossal waste of time because if you know they're there, you're going to cut them off (i.e. wipe the drive and reinstall the OS and/or change passwords, etc.).  If they were going to do something that called your attention to them, it would be to hold your computer/information for ransom; in other words install ransomware.
Is it a Virus?
Unlikely. Again, this would be a waste of time for a virus writer to infect your computer to change your background.  And technically, it would be called malware because a virus is a type of malware that self-propagates much like the flu.  Those are few and far between now and most malware is self inflicted (by downloading/installing software of questionable origin).
What do you do?
If you're concerned someone has access to your computer - change your passwords. Change the passwords on all of the accounts on your Mac.  If you've got three, change all three.  Change your iCloud password.  Change whatever account password you feel may have been compromised.
Now, if you Netflix account uses the same password as your Apple login, well, shame!  Get a password manager and start using it put different passwords on every online account.

Keeper Security (Free for single device, $30/yr for unlimited device and passwords)
Dashlane - (Free for 1 device and 50 accounts, $60/yr for unlimited devices and passwords)
1Password - (Free trial, $36/yr for unlimited passwords, items and 1GB secure document storage)
KeePass (Cross platform and free but requires a bit of tech savvy to sync across all devices)
McAfee TrueKey (Free for 15 passwords/unlimited devices; $20/yr unlimited passwords/devices

Still not certain you have malware installed?
Install security software like Malwarebytes, Webroot, or McAfee Security
However, if you do have an infected computer, I've always subscribed to the &quot;nuke it&quot; and reinstall philosophy.  This is where having a backup really comes in handy.
What about my changing background and open pictures?
Check your desktop settings - see if set to rotate (change every so often).  Change it to something else or turn it off altogether.  Then come back and reset it to what you want.  As for your open pictures, you may have inadvertently opened two pictures at once.  It's happened to me while using the trackpad; it’s happened to all of us at one time or another.
Keep your software updated

Every once in a while I see a message on the upper LHS telling me that
I should update something related to the system but I always click on
later and then the message vanishes.

If it's the left of the screen, it's not coming from the Notification Center.  Track down what it is and either uninstall the software (if not using) or update it.   Know the software you've got installed.
If it’s showing up on the right side of your screen, per your edit, check your notifications!  This is like taking your temperature, ignoring the reading and then jumping to the conclusion that you’ve got Hodgkin’s Lymphoma (it presents with a fever).  The point is, read and respond to the notification, that’s why Apple implemented it.
Probably the most controversial thing I'm going to advise here is that you should stop (if you're doing it of course) looking for and putting a priority on free software over known titles/developers.  Nothing's free.  It's much better to install an app that costs 5 dollars/euros/pounds/seashells from a developer that registers with Apple than finding a free app that requires you to bypass Gatekeeper to install.
IT is my profession for over 30 years now and even though I posses the skills to overcome malware, I will opt for the paid version over the free one 99% of the time.
Bottom Line...
If you skipped over everything above and take only one thing away from this it should be to have a backup.  Catastrophic things are going to happen - I guarantee it.
Even if you actually get hacked and your data is being held for ransom you can recover everything simply by wiping your computer, reinstalling, and restoring your data.
A Time Machine backup is the cheapest insurance policy you can buy.