ObjEffect_SetTexture(obj, filewiththeimage);
filewiththeimage is a variable you define earlier in the task or in script_enemy_main
For the latter part,
ObjEffect_SetVertexUV(obj, 0, 0, 0);
ObjEffect_SetVertexUV(obj, 1, 90, 0);
ObjEffect_SetVertexUV(obj, 2, 90, 88);
ObjEffect_SetVertexUV(obj, 3, 0, 88);
should work, I think.