If it's over 20MB or the size set in config.php, it won't show a thumbnail, as stated above.
Why can I upload an image, even though it is larger than the maximum file size (KB) permitted?
Uploaded files are first resized based on the maximum physical dimensions (pixels) set in the ACP -> Options -> Attachments: Maximum Attachment Image Dimensions. If the resized image is below the maximum file size (KB) limit, then it will be allowed.
Note that with XenForo 1.2 and above, by default, images of 20 megapixels and over will not be resized; this is to prevent fatal errors due to running out of memory. If the Maximum Attachment Image Dimensions combined total (width x height) is set lower than this limit, images greater than 20 megapixels, or with a dimension larger than that permitted, will be rejected with an error stating The uploaded image is too big. If the Maximum Attachment Image Dimensions combined total (width x height) is set to 0 or greater than 20 megapixels, the image will appear as an attachment without a thumbnail. The 20 megapixels limit can be changed using a config.php switch; refer to this section of the XenForo Manual.