It's been a while, but you'll need to install Erica Utilities and then SSH in to your phone with a screenshot command.
You should be able to find instructions on how to do that here with a quick search.
I think the issue here is Dock may not be available at the Lock screen, which would otherwise be the easiest way to take a screenshot.
Since you've obviously using a paint program, I would simply advice you to create your own mockup image of the lock screen. When I do my button mods I post my screenshots from a mockup springboard. Makes life easier.