close photo pop-up?

Sorry, I misread your question.

That sounds like it may have opened a Quick Look window. The close button is in the top left but depending on size can not seem like a close button. View and edit files with Quick Look on Mac - Apple Support

If Quick Look opened the photo then stopped responding I think restarting your MacBook Pro may be the only way to get rid of it, since its not an app that you would be able to see in Force Quit.

[Edited by Moderator]

If you click anything outside of the pop-up photo chooser thing it should automatically close. If not you may need to close the Messages App or select the Apple Menu to force quit the app then when it reopens that should defiantly be gone.

I hope this helps.