1) it is too big (the limit is 2MB, but we kindly ask you to try and keep all your images uploaded to the server, including your photo albums, under 1MB).
2) or - most likely - it is too wide. Please keep images uploaded to forum under 2000px, otherwise they might not upload.
Actually, I cannot figure out the strict limit, some pictures upload at 2100px, some wont upload at 2000px, but just try to keep a reasonable width and they will go through, there is really no need to upload a huge pic 2000px wide!