Something like the Gas Cloud in this set: <a href="https://marketplace.roll20.net/browse/set/1198/leg" rel="nofollow">https://marketplace.roll20.net/browse/set/1198/leg</a>... 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.