Making a drawing that looks like someone else's is not illegal.
Actually I am not sure about it. Maybe it depends on how similar they are.
Actually it's only meant to depend on whether the later work was actually derived from the previous one (with very little changed to constitute an original work). Due to uncertainty, this becomes a question of whether it's
provably derivative. Also, a derived work can be permissible if the intended effect is markedly different, such as with parodies (see 'Fair Use').
The only real difference between these two images is where the tremulous one has had the circle resized and positioned. Everything else I'm sure is derivative with only minor editing (outlining, blur and colour; see attached differencing image) so unless you can find some mutual public domain source, this is an infringement.
I agree with vcxzet in sentiment. I don't feel it's likely to cause legal issues but I do feel it should be changed for the principle of it. It's a little image so it's potential threat is somewhat matched by the ease of replacing it.
So, though I have no experience making icons, here's my contribution as an alternative:
