I watched an interesting video on Youtube, showing how iD software re-used assets in their sprites for Doom and how some monsters contain bits of other monsters. Working quickly and efficiently is a top priority for game developers, especially if you work on your own.
I needed a power-up graphic and inspired by this video I decided to rework something which already exists. Of course all my graphics are just downloaded from various places, not drawn by me! In a commercial release this would not be possible but for prototyping I think it's OK to 'borrow' graphics as this game will never be commercially available.
I took the sub which I am using for the player sprite and after a few quick tweaks I had something which looks OK and hopefully the player would never notice.
Here's another example of tweaking existing art.