It’s in the game’s profile folder, either /Android/data/bruenor.magicbox/Games/Data/[cryptic profile name]/Screenshots (private location), or in /.MagicBox/Games/Data/[cryptic profile name]/Screenshots (public location).
A bit difficult to find, if you have many games installed. The easiest is to save your profile without any changes in Magic Dosbox, then sort them by date in your file browser, so the most recent edited folder & file is the topmost.
That being said, I realize you’re the first person to ever ask this question, and I think an export function might actually come in handy. Or maybe the screenshots should be stored to something like /DCIM/MagicBox Screenshots/[Game Title]/ ..? ?