Questions answered by this recipe
How do I add a new image type extension to the list that PmWiki automagically recognizes to display as images?
To add a new image type extension, say .ico (icon), you can append the extensions to the $ImgExtPattern variable in the
$ImgExtPattern = "(?:$ImgExtPattern|\\.ico|\\.ICO)";
An example of adding .bmp (bit map) and removing .svg (that is enabled in the default installation) requires to redefine the variable:
$ImgExtPattern = "\\.(?:gif|jpg|jpeg|png|bmp|GIF|JPG|JPEG|PNG|BMP)";
See discussion at AddImageType-Talk