Keywords: opacity, roofs
(See the best response by Kaiochao.)
Is there any way to display an image over top of or through opacity? I'm trying to create a roof that will be visible on buildings with opaque walls.
Would this work?(Just what you could use)

sight var (mob)

SEE_INFRA // can see infra-red objects
SEE_SELF // can see self, no matter what
SEE_MOBS // can see all mobs, no matter what
SEE_OBJS // can see all objs, no matter what
SEE_TURFS // can see all turfs (and areas), no matter what
SEE_PIXELS// if an object is located on an unlit area, but some of its pixels are
// in a lit area (via pixel_x,y or smooth movement), can see those pixels
BLIND // can't see anything
No. I've tried all of the visibility flags and they don't alter the default behaviour related to opacity.
Best response
Setting the layer of an overlay to 0 would work, but it would need to be attached to a visible part of the building. I use a screen object to capture mouse clicks made in opacity.