MATERIAL definition controls the "lighting" properties of an entity or surface. u cant assign an image to that. usually the definition of material is the ambience, specular and diffuse lighting properties. see manual for MATERIAL definition.
instead you try this (not tested):
BMAP* bmpskin1 = "skin1.tga";
MATERIAL* mtlskin1 =
{
skin1 = bmpskin1;
...
...
}
if (case)
{ent_mtlset(my,skin1,1);}
u can replace the "..." with other lighting properties.