Minecraft wiki entry for Creating_resource_pack_add-ons, associated text: Make sure it has the same name as the asset you are trying to replace
Example: Creeper texture should be called "creeper", the stone texture is called "stone"
Make sure the texture is in the correct folder!
Ensure that all image files are in the right format (.png, .tga, or .jpg) or they will not load.
Ensure that the image files have been saved as the right extension, e.g. don't just change the extension. If using GIMP, use the "Export As" function (under "File").