I always copy any image into LibreOffice Writer, then copy from there to the forum. That process also usually converts .jpg to .png as well. (Or so I think…?)
(I initially started using Writer so that I could compress images slightly, but then stopped compressing my images at some point.)
When I need to alter an image, I open it up in GIMP first, make the alterations, then export to .png. Never had a problem uploading these.