On my phone, i sometimes use #ImagePipe to downscale the image, crop, and reduce quality, works well for me #FDroid
Some points of improvement:
* It could be handy to share a bunch of images to ImagePipe and add a task that's a list of images which can be shared on, again as list.
Now i tend to just write a bunch of files to then share as list again
* i find the cropping interface awkward, have to hit undo a bunch.
I should add, a resolution and quality dropping doesn't matter for many uses for the viewers, but saves a _lot_ of bandwidth and storage. Like ×20