Threw this together real quick the other day, but it had the ol' HL1 pure blue for transparent background and was kind of an eyesore.
Reworked it real quick tonight. I know they're simple and rough, but I'm no artist... Hope some of you like 'em:

To use:
After downloading the .tga file, click on the "import spray" button from the in-game options and simply navigate to the .tga file that you want as your spray! It's that easy...
And if anyone wants to rework these to something nicer, feel free!