You need to place your background in a higher video timeline. It is a little counter intuitive but think of it as a stack of paper, first one you lay down gets covered by the next one and so on.

Thanks. So if I understand you correctly, I need to insert another video timeline above the existing one which has all the clips. ASUS P8Z68 deluxe
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You should already see several timelines available to you.

Place your background in the first (that's the bottom of the virtual stack).

Now if you insert another item, still image or video clip on the next track, PDR will firstly 'try' to fill the frame with it but the aspect ratio will be preserved. If for instance you place a 4:3 picture on top of a 16:9 one then you'll see two vertical strips on each edge from the underlying one.

Your background will either be fully or partially obscured by the topmost item. You can show items in lower layers in a number of ways, by resizing overlaying ones to be smaller or by making certain colours transparent (Chroma Keying) or in a similar way by using items which have a transparent colour (alpha channel) inbuilt and enabled as part of the file type.

The 'stack of paper' idea is still a good way to think of it. The top sheet covers the lower ones unless you cut out a hole in it somehow.