Any file which will load into Movie Studio can be used as a background, just add it to the timeline by dragging & dropping or use the Import tab next to Effects & Templates, then place that image on track 1.
PS. if you want to create your own background that will show in the Background option in Templates, -
Go to - C:\Users\Public\Documents\MAGIX\Common\Videocontent\Movie Studio Platinum Content\VideoEffects\BackgroundEffects\Pictures - add your 1920 x 1080 jpg,
In that folder is another folder called Icons, open that & add a reduced size version of the same jpg, make it similar size to the others in that folder, 600 x 337 (1920 x 1080 ÷ 3 = 640 x 360, that size will work fine),
These two files, the bigger 1920x1080 one you put in Pictures & the smaller one you put in Icons both have to have the same name, Restart MS, they should show & will apply as a background like the others do,