ok, it's not quite PhotoShop but you can design your livery online...

Here's what you do...

Download this file: PSB Template

Go to: http://www.pixlr.com

Select: Image editor at the top of the page
...the app will then load

Select: Open image from computer (select the PXD file you downloaded)

Now select layer 1 and paint away...

When finished save as a jpg and you can upload here in the forum...