If you host the photo elsewhere, there is no limit, except a practical limit. The practical limit is that it fits for most screen sizes. There's nothing worse than having to scroll to see an entire photo. My photos use a 2x3 aspect ratio, and I generally use 1000 pixels as the longest dimension. For landscape orientation, that means the photo is 1000 pixels wide.

If you upload the photo to the forum, using the File Manager, the limit is 250K.