What error message? You can post most image formats (JPG/PNG/GIF/TIF...) up to 500K, and XCF/XCF.GZ files up to 2MB (XCF.GZ is a compressed form of XCF (you can use that extension when saving the file) that usually gives a 3:1 compression ratio).
If the image is above these limits, put it on imgur and add a link to it, or crop it to some the relevant/meaningful extract before posting here.
If the image is above these limits, put it on imgur and add a link to it, or crop it to some the relevant/meaningful extract before posting here.