In Android, any image will show in the gallery unless a blank file called ".nomedia" is placed in the directory where the pictures reside. Likely, these are images used by a particular application, but the developer did not include this .nomedia file. I am not sure, but I think you could just open notepad, create a new file, and save it as ".nomedia" with the type set to "all files." Then copy and paste this wherever you need to.
Album art will automatically hide from the gallery if the .jpg images are named albumart.jpg. If they are named anything else, they will show up. I don't know how your music is sorted, but I have all of my albums in their own folders, and the jpeg of the album art is inside the folder named albumart.jpg.