File size is ALL that matters. It HAS to be a JPG file. No PNG, it will vanish.
This image has lots of pixel width, and is just compressed to be under 5 MB.
Pelican resized it to work.
Just click the "GO Advanced button" and that brings up guess what, the advance posting options, click the Click Here to Upload Photos button, and just drag and drop you photo to the window. TA DA! Click the Submit reply button, and done.