Mmmmmmm. Dunno
Its a bit of a grey are for me too. I know, the admin......is meant to be the font of all knowledge ...but i'll need to check it out.
the max image file size for images in posts was 128kb with max length or width or 800px, but I updated to 200kb per image, max of four (posts only).
I think you may have posted just after i connected the gallery to the posts ans d possibly prior to the increased max file size.
try another image (or the same, but keeping below the 200kb limit per image)
