To Change an Image just convert your image to a DDS (easily done with Paint.net {GOOGLE IT}) then place the DDS image in the Textures folder and reame it to match EXACTLY with wich one you are replacing! (I.E.) if you are replacing the how to photo you would name it "20" nothing more!

Note 1: unless you have experiance with  modding DO NOT change the photos in the Icons Folder!!! if not done correctly it will break the mod!! 
Note 2: when you update the photo in the Textures Folder IT WILL NOT update in the store!!