I can't figure out how images work on XF...
Images don't have actual URLs. For example, the one I'm attaching, which was called cane-che-gioca-palla.jpg, has become
You've posted the URL, Google has no issue crawling or indexing them if it's deemed worthy. Google (and browsers in general) use the Content-Type header and other context clues to determine the resource type, regardless of the file extension in the URL itself (or lack thereof). In the same way Google knows the URL to a thread is HTML, it can reliably determine the URL to image attachments are images.