I'm testing RED first time and just wondering where there is a possibility for background images. Wether for body nor for other elements is a background image option available. Also I wonder why, if onetime is activated a background color it is not possible to set it back to transparent.

Can anybody help?

Yes you can use images in a background.
Go to the design tab and scroll to the background properties area. Now select Resource or url from the dropdown next to the word Image