Thumbnail images don't show in media gallery

I have been having this issue for a while and am not sure what to do. My thumbnail images (preview) do not show up in my media gallery. They are all just boxes with question marks in them. I have tried deactivating all my plug-ins. I have created a child theme (still not sure how to use this really) I have checked my cpanel for permissions (755 files - 644 folders is the norm). I have contacted my hosting site to see if they could help and they cannot. Any suggestions?