The file you are uploading is too big!
Either its resolution is too high, or its filesize is too large.
If your file is over 300KB, please compress it. TinyPNG is recommended for PNGs and JPGs.
If, after compressing, your file is still too big, or your file's width times height is over 1,049,088 pixels, please resize your image to under that pixel count (but do preserve the aspect ratio).