Improved XCF rendering was developed in T37622, but is waiting on renderer software being upgraded (see T65899). Thumbnails may contain errors.
In particular, indexed images are not supported and must be converted to true-colour or greyscale in order to be displayed. Additionally, correct rendering can be obtained for some images by flattening the image (e.g. in GIMP) and reuploading. Keep in mind that this technique dissipates the advantages of XCF (e.g. Layers).
Note: if the screenshot shows any work that is not a direct result of the program code itself, such as a text or graphics that are not part of the program, the license for that work must be indicated separately.