[Brian]

They're stored inside the application package. You'll want to open it up, navigate to
Contents/Resources/
and then copy the PerspectiveImages folder out to your desktop. Modify the copies and use them; the folder inside the app package will be replaced when you update OmniFocus in the future, so you'll lose any changes you make inside the app itself.

(I'm deliberately being a bit vague here, because I don't want to encourage folks who don't know what they're doing to mess around with files inside the application.)