For comparison, a 900 pixel photo.
Total width in pixels of the post area is 732 pixels wide. If your photo is wider the right side will get cut off.
In the old days, when we were running an earlier version of the forum software, a wider image would "push" the page's right side out to match the width of the photo -- for everyone, all the way up and down the page -- and novice users could unknowingly post very wide images to make topics unreadably wide (until a moderator edited or deleted it).
Now, is this to say there is no way to post a larger image here? No! You can post a link to any larger image... 10 thousand pixels wide if you'd like.
If you use the Upload attachment feature that lives just below the posting box, you don't have to worry about any of this.
