A Microsoft file hosting and synchronization service.
Hello,
Sounds like your account is fine and was never actually hacked. The mobile app displays local pictures found on your device in addition to what is saved in your cloud. Many mobile games and communication apps download thousands of temporary ad images or files behind the scenes and your gallery is likely pulling them in. Since you already checked on your computer and saw nothing, that confirms these files are purely local to your phone storage and never made it to the cloud.
Can you open the app settings on your phone and look under the camera backup or device folders section? I want to know what extra device folders are showing up in that list.
Once you tell me which local folders are showing up there, we can figure out exactly where the junk is coming from and I can tell you how to wipe them out all at once or how to disable the app completely if you no longer want it.