In general we source, color correct, and (if possible) upscale non-photo images. There isn't a minimum resolution per se, however if your image contains modifications that you made personally, please make sure to use the highest quality source possible as we will not be able to do anything but color correct it. Keeping this all in mind, it's a good idea to use something at least 1400px wide and 1000px tall. Bigger is always better and there is no max file size.