When issuing for playbin the convert-sample signal :

1. JPEG and PNG work but produce invalid files because an unnecessary header of 14 bytes is added at the beginning of the image. This header looks like the header for a .bmp file. Here is one such header for example:
42 4D 9B 20 01 00 00 00 00 00 36 00 00 00
If the header is erased, then the file becomes valid.

2. Other formats don't work. The GStreamer List of Defined Types contains also image/gif and image/tiff, but they don't produce any sample (zero returned).