The following samples test various forms of using the image&tile option in the BACK chunk.
The first two show a static background composed of a tiled image.
In the left sample the entire canvas is covered with the image, and in the right,
clipping boundaries were used to limit the background image to a smaller rectangle and show
the BACK color for the encompassing frame.
If the samples only show a uniform background color (possibly with a tiny image near the upper-left corner),
your MNG decoder does not handle BACK image and/or tile operations.
This will be the case for implementations based on libmng-1.0.4 and older.