there is a pre-made one included in the CS:S .gcf, btw. search for 'parking' in your texture browser.
Edit: oh, duh. just occurred to me that you might not want a parking decal.. hmm. well, anyway, it's there. :cheese:
yeah. the one I know off the top of my head is 'vtf explorer' and I'm sure there are other.
just browse to the counter-strike shared content something something .gcf file, it's at...
hmm.
counter-strike source shared.gcf -> cstrike -> materials -> decals -> handyparking_e, n, s, and w.
heres a thumbnail:
Edit again: Oh, and I noticed it doesn't seem to have much transparency going on anyway