That's because the forum software code is set to take images only up to so many megabytes. Original camera images can be very big and the software resizes them. An original camera image could be 10 or more megabytes, depending on how much of the image tends toward darker shades. The reworked image on the forum might only be 100 kilobytes.

The best approach is to edit the original, save as 620 pixels wide. Then the forum code won't touch the image and you will never have a problem with rejection of giant images.