Assuming that you downloaded and installed the Extra Content, Backgrounds are under Templates tab, Image Objects, Backgrounds. You can always find other backgrounds on the internet or make your own.
There are other ways. What is the purpose of the background? Ex. to go with a portrait video clip in a landscape project?
Snapshot:
If you mean that you want a still image on the timeline from a video that is on the timeline, place the playback marker, right-click, select Insert still image at the playback marker. You can modify the duration like any image.
If you mean that you want to export an image from a video clip to a file, then, place the playback marker at that location, top menu File, Export movie, Single frame as jpeg.