So i recentely made art for one of my horses, but whenever i go to put it on the horse it says invalid file, i don't know if its to big or if its just the wrong type of files. could anyone help?

If it is, the file size is too large. You're going to need to use an image compressor to compress the file to under 100kb [they can be found online or typically within your art program].

Ah i use gimp, i am not sure what a image compressor is though