SC2Mapster

I believe you may have to specify it through the text path something like

assets\images\mynewimage.dds

I know Mephs does a ton of custom images in his maps.....

this may be relevant.... http://forums.sc2mapster.com/development/triggers/15108-importing-images-with-transparent-backgrounds-into/#p9

this says you should see it available after you import then save the map. Possibly try reloading the editor after saving

http://forums.sc2mapster.com/development/map-development/3713-custom-images-importing/