ImageMetadata.jl looks like it’s for images already loaded into Julia but the documentation is pretty sparse so I can’t be sure about that.
This method is only limited to whatever exiftool is limited to. exiftool does work on a variety of image (include vector image formats) so depending on the format you want to use it is possible. See this list of formats that exiftool can deal with. I successfully tested writing and reading tags to an EPS image.
I hadn’t thought about overloading the standard save functions but it’s a good idea; anything to overcome the laziness and forgetfulness that limits how much metadata is saved. I’m a bit torn if it should be default because of the added save time and I haven’t dealt with Plots.jl yet so I’m not sure how it would be implemented.
Picasa is still quite functional if you deal with lots of images but it might not deal with tags from much other than JPGs. It’s part of why I’m looking for something better.