Any photo editor will do. Just load it in and export as JPEG or smaller resolution.
On f-droid:
Pocket Paint is crashy for me, but it gets the job done.
Imagepipe has a bit clunky interface but just to load and export it works fine. Its primary advertised purpose is to remove metadata from photos, so not a bad thing to have on hand anyway.
Any photo editor will do. Just load it in and export as JPEG or smaller resolution.
On f-droid:
Pocket Paint is crashy for me, but it gets the job done.
Imagepipe has a bit clunky interface but just to load and export it works fine. Its primary advertised purpose is to remove metadata from photos, so not a bad thing to have on hand anyway.