It is not always a lossy conversion, e.g. saving a GIF image to a PNG image is a lossless conversion. If the operation is lossy then show a warning (assuming yo

It is not always a lossy conversion, e.g. saving a GIF image to a PNG image is a lossless conversion. If the operation is lossy then show a warning (assuming you haven't already shown a dialog like GIMP's file format dialogs), if the operation is lossless then there is no need for such a warning. Similarly if you are editing an image with a single layer (the background) and save it as a PNG it is always lossless (no need for warning dialog) but if you add a layer and try to save it then it becomes lossy (so show a warning dialog with the option to proceed as the default - being default because that is what the user asked for - and an option to save as XCF instead so that the layer is preserved).