I have several memory questions regarding OS X 10.9.5 I have just purchased CS3 and it has been legally transferred to me. Do I have enough memory to handle it? In addition I want to upgrade to Yosemite or El Capitan. Do I have enough memory to do that too?

Also, can I erase photos from iPhoto that I no longer need and will this give me more memory?

Attached is my memory indication.
It looks like you're running Mavericks (10.9.5) based on the screenshot you provided. That being said, why would you want to downgrade to Snow Leopard (10.6.x)? Yes, it is probably a touch faster than Mavericks, but it isn't supported anymore and doesn't have the memory compression feature that might help a bit for a memory hungry application like CS3.

Either way, you have 4GB of RAM which is enough to do basic things on the machine but with CS3 it is probably going to be a potentially sluggish experience if you use it heavily and have more than one application open. I would suggest upgrading your RAM to at least 8GB if you can. What kind of Mac do you have?