no, fuck paint.net. Photoshop ftw.
Go edit your alpha channel...
you probably didn't and it's still in the same shape as the old scope, meaning (since it's the transparency channel) the shape of the old scope will still show through regardless of you deleting it or not.
on the layers tab go over to channels, click on alpha, use black and white for it. make it the same shape as your scope.
black = transparent (it wont show up on CA)
white = visible (it WILL show up on CA)
if you look at both the RGB and Alpha channel the part that wont show up will appear in pink/red. make sure all the edges of your scope are red, use a brush. otherwise you'll have a white outline all around it.
Report results here.