I use Visualize for Rendering so it has been a little while for me.
Did you set the image masking?
Under advanced on the left side; I believe you can set the masking to pull from file (PNG) or you can use the little dropper tool and tell it what to mask.
Even if you drag and drop a decal, you should be able to edit it. Unless it is coming in as a sketch picture or something other than a decal.
Go to the decal tab, right click and hit add decal, then browse for your image and check to see how the masking works.
thanks for your response.
I was able to edit, by dragging a default decal then going to browse and selecting the decal I wanted.
Secondly, I had to enable 'Use decal image alpha channel to make it transparent.
All resolved thanks for your help!