Android

nomedia

When ads make in into gallery - what you're seeing is probably cached images from one of those apps. Apps that aren't very well written won't add a ".nomedia" file to the directory where those images are cached--".nomedia" instructs the system to ignore that directory when looking for photos to add to the Gallery database. This kind of thing is fairly common, even among legitimate apps--it's generally not malware, and isn't necessarily adware either.