I made a token for my game, then deleted the image from my computer like the silly I am, and now I need to change the token, but I don't have any access to it. Can I save it somehow from my game/library?
As far as i know you can only upload images from your computer to roll20. You can't download images from roll20 to your computer. I would suggest that you go on the web and see if you could re-download your token or see if you can restore your token from the trashcan on your desktop.
Check out this thread for a snippet by Aaron (and some suggestions by me) for a means to download any of the images in your art library. I assume it still works, but I haven't actually tried it since the changes to the art library layout.
I found a work-around for this. Bring your token onto a map and select it. Press "Z", which should bring a zoomed in image onto your screen. If you right-click on the zoomed image (using Chrome, I'm not sure about other browsers), you'll get a menu with the option to save the image.
Alternate method, if Z doesn't work for some reason. Just set it as the Avatar of a character (open character sheet, click Edit, drag the image from your art library), and Right-Click Save-As once it is set to download.