Here is a list of known issues of Mapserver affecting Cartowev.
Mapserver 4.8.1 & CartoWeb (Any Versions)
Size of SYMBOL of Type PIXMAP Can Not Be Set
In the latest version of Mapserver, one can not set the size of a symbol of type PIXMAP (gif, png, ...)
The CVS version of Mapserver has been partially fixed, as you can only controls the largest pixmap dimension, either the height or the width.
Partial solution: compile Mapserver from the CVS.