I use Apple computers, but others would have a similar function to this: with your image opened on screen, open File; from the dropdown menu choose Export; then on that form against Export As, rename it slightly; where to save it (Desktop), the format (JPEG) and move the Quality slider so that it is less than 500 kb; then press Save. You should then be able to post the result to Rootschat.
Hope this helps.
Peter