Image posting size limit

I'm attempting to post an image file, in the .png format. I took pains to make sure that it wasn't over the 5mb limit, and I got it down to 4.5 mb (4,586 kb) or so. However, when I try to post it, I get the same error (not within limit, wrong format, or interrupted connection). I know it isn't the connection, and the image size is within the set parameters. Is there something else I can do?

At 5 MB it should work. I would try to get it smaller and see if that works. Alternately, you might want to convert it to JPG. Occassionally PNGs have issues.  - Gauss

How big of an image is this? Even a high-resolution display like the retina macs which displays at 144dpi and with poor PNG compression a 4.5MB PNG would be almost 23" x 23". More realistically the default grid size is 70 pixels/inch which would mean a 4.5MB PNG would cover approximately 47" square.