Upload & Sell: On
If you get that message when you try to post one, you have two choices: Either save it as a smaller file (either by reducing the pixel dimensions or the JPEG quality), or upload it to some web site like flickr, zenfolio, etc. If you paste the location of the image file into your post, instead of doing a direct upload, that file size limitation isn't there. I usually make them 900 pixels on the long dimension, and that often, even usually, results in a file too big to embed directly.
If that's not clear, ask; there are many members besides me who can explain further...