Questions answered by this recipe
How do I add a new image type extension to the list that PmWiki automagically recognises to display as images?
To add a new image type extension, say .ico (icon), add the following line to the
$ImgExtPattern = "$ImgExtPattern|\\.ico|\\.ICO";
An example of adding .bmp (bit map) is
$ImgExtPattern = "\\.(?:gif|jpg|jpeg|png|bmp|GIF|JPG|JPEG|PNG|BMP)";
See discussion at AddImageType-Talk