For some reasons, the SMF 1.1.x thumbnail generator, which is used when someone upload a large image, cannot handle extremly large images such as a wallpaper. This seems either like a bug or server-side problem. When trying to upload such large images, the topic will give a 500 Internal Server Error message until an admin removes the attachment from the Admin panel. Because of this, we decided to disable the thumbnail system, meaning when uploading such images, they will no longer be resized (nor requires you to click to see original version). You can still upload attachments fine, except that with massive ones, an horizontal scrollbar will appear in your post and topic might be harder to read, especially for dial-up users. It is getting a bit harder to do compromises in this Web 2.0 era, though.