Yes, we do have to limit the size (dimensions as well as file size) to maintain the integrity of the message board. You'll need to use a utility (Paint, or Photos, for example) to reduce the size. From another thread -
Photo uploads are limited to 1Mb each - and that allows for a HUGE photo. However, please be frugal and post photos of an appropriate size (for viewing and storage). HD resolution is 1920 by 1080 and so the max resolution we currently support is 1920 by 1920 (to accommodate both portrait and landscape orientations of HD content).