Hmm.. I find the whole of Construct's sprite drawing rather buggy to the point of being barely usable
So, here's what I do. I draw all my sprites and animations in MMF2 and copy and paste or import them into Construct.
Maybe you guys could check opacity, I found out that most of the tools weren't drawing because they were somehow set completely transparent.
But really, I think the picture editor is Construct's biggest flaw. It's best to use a 3rd party one. Even MS Paint isn't half bad if you get used to it.