XCF rendering is currently under development and thumbnails could 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).
English: One of the early graphics cards from ATI Technologies: a Hercules card from 1986. As can be seen from the PCB the layout was done in 1985, whereas the marking on the central chip CW16800-A says "8639" meaning that chip was manufactured week 39, 1986.