in (html-)full-message-view, you can select text with a long-tap and copy it to the clipboard.
but you currently cannot get out an image.
the issue is not super-urgent as most images are shown also in the normal chat, however, for the few that are not shown there, it would be nice to have an option beside screenshot".
i suggest to show a simple option-alert when long tapping an image with at least the option “export image” and “cancel”. might be that there is already something built-in in
WebView, that does this job, maybe a bit differently - if so, i would prefer that. whatever is simple