To prove, you need unambiguous traces. You need to find out what software capable of opening the image is or was available to the user on that computer, and you need to find out if that software leaves such traces. And you usually also need to establish that such traces are not left under any other circumstance than through opening the image using that software.
I assume you're referring to 'person' only as a convenience. If you're not, you may also have to establish who was using the computer or the computer account at that time.
As for accidentally vs. deliberately ... well, that's a different can of worms.
To determine if someone opened an image, you could look through LNK files as well as MRU's for the file extension that the picture was saved in and software tools installed that are capable of opening files. I would suggest parsing the registry hives / link files and searching across them for your filename of interest