[eluser]Colin Williams[/eluser]
$300 is tough to justify as an expense? That's what, 3 - 5 hours of billable work? I guess you have a few options. You can start with a 30-day trial of whatever Adobe software you choose, and just quote in those extra hours to cover part or all of the expense. So, you have 30 days to finish the project until the license runs up, at which point you should have collected at least half of the total bill, which you can put toward purchasing the license.
The other option is to use The Gimp, open-source photo-editing/digital-art software.