Well, not knowing exactly what your art looks like (ie how much of the page is covered with "paint" or color) --
-- IF your picture is only transparent for the highlights, you can make a new layer under the one with all your good work on it, (it will probably appear on a layer above, but just drag it under in the Layers Palette), and you can fill it with white.
-- OR you can make the new layer and broad stroke it generally under where the highlights are thus leaving the other areas of the background clear to be painted on -- on yet another separate layer below.
Once you have the concept of how it works, it should be easy for you thereafter to use layers for all kinds of stuff. This time it's a band-aid. Next time it's for massive effects or mats with scratches (or who knows what).
Layers are God's own art creation tool. If there is stuff on separate layers in your pic, you can adjust them infinitely because they're isolated already and it's there for you to play with to taste on the fly after the fact (tweak colors, adjust transparency etc etc etc). And of course, that's just a bit of how they can be used.
Have fun with it.
Oh, and welcome!














"Not a bit is wasted and the best is yet to come. . ." -- remembered from a dream