I’ve looked everywhere on the Mac and can’t see an easy way to do it , I end up having to go out of full screen which kind of defeats the purpose tbh …
If the image is in your Photos app, you can get to it by tapping the little Apple/"A" icon, then the Photos icon. If it's on your Mac disk, from the menu bar tap Conversations, then Send file.
right-click/Control-click in the text entry field. 'Import Image' from the context menu. This opens a 'Select a camera or a scanner' dialog. However, it doesn't give you access to a file browser.