I tested this out as a student in a demo course and was able to paste photos into the RCE that were added to my computer's clipboard. If you're not able to paste photos into the RCE, I'd recommend verifying photos you are copying using the snipping tool and other tools are saving to your computer's clipboard before you attempt to paste them into the RCE. If they are saving to your clipboard, but aren't pasting for some reason, I'd recommend reaching out to Canvas Support for further assistance.