Where is the best place to post a request for someone to do one off Photoshop jobs?

If you specify what the task is its easier to answer - for example whats the complexity in terms of steps, what fluency in photoshop does it require, and do you need an english speaker to interface with. Joona is right in the fact that you can easily hire 15 minute workers off http://freelancer.com; however, for photoshop designs this is a poor outlet, in my experience, as the designers on there are just terrible. If your job is simply just following steps and has a short learning curve, this could work though. Throw up a project (its $2) and see if you can establish an ongoing relationship with one of the workers. Same goes for sites like http://PeoplePerHour.com More appropriately you could try http://oDesk.com as it has the built in team room panel. The team room allows you to monitor the progress and productivity of your worker and allows you to only select the ones that are available when you need them. This would be a problem for http://freelancer.com as alot of bids are just automatic and will never get a response if you message them. You can also cut off your workers when they start deviating in quality - no messy escrows like http://freelancer.com. If you require a fluent English speaker, or can afford to work with one, a great way is to just post your gig on Craigslist under "Gigs" > "Creative". You'll get a dozen of responses in half an hour (tons of people just need side work). You'll actually get to see real portfolios; if you want to minimize costs post in the Midwest hubs as the comparable wages are significantly (40%) lower then the coasts.

Have You tried http://www.freelancer.com/ or http://freelanceswitch.com/? As a designer I'd recommend that You sign a clear contract (based ie. on hour tax) with a designer that You can assign Your work regularly.

If you are looking for a quick photoshop work you can take a look at this gig on fiverr http://www.fiverr.com/s/5km66i