Since some Aple updates ago I can't post screenshots either. They are exported as .PNG files, I believe.
I change them into .jpeg files by exporting them to a free app called SketchBook X (by Autodesk) which exports them back to my photo-library as .jpeg files. Those files are a smaller size, and that seems to help.
But since SketchBook is a photoshoppy thing......your screenshots will not result in someone being found "proven guilty beyond a reasonable doubt".