If you you just need to find gimp on your system in order to tell the appimage where it is located, then you can user the
whereis gimp
command in your terminal, It will show you most likely /usr/bin/gimp as path.
I think your primary issue is that the appimage is not integrated into you system and thus doesn’t read the installed applications from the .desktop files. Best is to install digikam via the package repositories… Big warning, I’m neither a Mint nor an appimage user.