Creating overlays

im having problems creating overlays like smoke or darkness and the like am i missing something is there a trick to it or is it a feature for pay levels. 
1504642705
The Aaron
Pro
API Scripter
When I've done this (for darkness) in the past, I created an image with transparency and dropped it on the map layer.  What issue are you running into?
im tring to create an overlay to represent a poison gas. but the back ground continues to show solid so it covers up the map instead over overlaying it. i might be doing something wrong on the image not sure 
1504643610
The Aaron
Pro
API Scripter
Something like the Gas Cloud in this set:  https://marketplace.roll20.net/browse/set/1198/leg... If you're drawing your own, you'd need to use an art program (like The GIMP) that supports having a transparent background, and set the transparency on the layer to some middling value, then save it as a PNG.  Discussion of that is beyond the Roll20 Forums, but that should give you enough detail to find it on a search engine.
thanks