I have no idea what that does, actually.
I'd use an app like Astro File Manager or ES File Explorer. Have it look for image files and video files and erase them all (after backing them up to a PC or uploading to Dropbox or something.) ES File Explorer, when you tap the menu control top left, has an option Library, then you can quickly find image and video files. I never connect my phone to my Mac, but I guess you could try doing it by connecting the phone to your computer and doing it from there. Again, you may want to back everything up first.
You want any image files or video files, I think. Wallpapers, photos, movies, videos, etc.