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).
The ODbL does not require any particular license for maps produced from ODbL data; map tiles produced by the OpenStreetMap foundation are licensed under the CC-BY-SA-2.0 licence, but maps produced by other people may be subject to other licences.