Thanks for your reply Gnome.
I'm a Mac user, and apparently the equivalent directory path would be:
X:\Users\<UserName>\Library\Application Support\Mnemosyne\
However, there is no Mnemosyne folder in there. Anyway, what I found was that
X:\Users\<UserName>\Library\Mnemosyne\
worked fine. The problem was that the image I was trying to access had the letter "ö" in it (knölsvan.png), which was the reason it couldn't open the file!