Backend of the app or the lemmy server? if it is not stored on the lemmy server then there will be no way to delete it even if the app stores the token.

Apologies, I worded that badly. Lemmy uses an image hosting service called pictrs to manage the images you upload, which is largely separated from the rest of the Lemmy backend. Pictrs of course stores the delete tokens matching each image, but Lemmy doesn’t associate those tokens with the posts or comments they originated from as far as I know.