Try this
steam - settings - in game - screenshot folder
Choose a folder to save screenshots in. You can change the shortcut key for screenshots here too.
In game press the screenshot shortcut key, exit game.
Copy from screenshots folder to X:\Automobilista\Plugins\DynHUD\editor\backgrounds\Reiza T-Cam\background_2560x1440
Open dynhud editor, Resolutions - choose your resolution.
BTW I had to make sure steam overlay was enabled to get the screenshot shortcut key to work.
Also, I find setting the place for widgets works perfectly if I change dynhud to use pixels rather than percentages.