The reason I ask is, most forums show a small thumbnail to guests to entice them to join and see the full size picture, but on a phone you can simply pinch and zoom the thumbnail to full size which defeats that.
What's the smallest size you can make the attachment thumbnails?
The default thumb size is 120px I think. If that's a e.g. 1200px photo pinching to zoom in isn't really going to give you the full experience. I guess you can make them even smaller if you're still concerned.
So, I set the thumbnail size to 25 and proceeded to rebuild attachment thumbnails, it made it almost through when it stopped with this error:
Fatal Error: Allowed memory size of 96321944 bytes exhausted (tried to allocate 21888 bytes)