It's probably some file that you can't see.
The fault above is regarding one image only, it generates two different errors.
First because its not a valid file (jpg, gif, png) second it can't find the file which is obvious since it wasn't created due to not being a valid file.
Look for hidden files in your images directory, some hidden file or just another file should be there. Or just an index.html file you created so people can't browse the directory.
Only image files should be there, if you need another file in there you can have it and ignore the errors produced.