Setting background image to dropbox file doesn't update background Im trying to have my desktop background be an image stored on my dropbox account. My dropbox folder is linked in my home directory. When i set the background to this photo and then change the photo the background doesn't follow. I know the file has changed by opening in Finder.

If I reboot the machine the image changes so it must be cached somewhere?
You're right, it is cached. So if you want to change the desktop image, you'll need to change the db file. In Yosemite, that is ~/Library/Application\ Support/Dock/desktoppicture.db

And here's a way to change it using the Terminal:

sqlite3 ~/Library/Application\ Support/Dock/desktoppicture.db "update data set value = '/path/to/your/image.jpg'" &amp;&amp; killall Dock


So, if you want to run that after a file have changed (such as modyfing it through dropbox) you could use a file watcher script, such as this Ruby one.