Like you, this first inspiration photo is my favorite and I think we could definitely duplicate this look for your stairway.  I'd love to use a mix of gold and white frames and some family photos as well as bright and colorful artwork and prints.  

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I love the idea of this more lined up/matching type of arrangement for the bottom of the stairs/hallway where you discussed wanting some additional items.  We could do a cluster of family photos for that area (either in color or all black and white).

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Below are some of my picks for the gallery wall to get us started.  This gold Target frame (below to the right) is a great option that is low-profile too (would also work for the hallway area) and it comes in a variety of sizes.    A lot of these bright prints/artwork I have included already have frames which is a plus.  

As far as your built-in shelves I really like all the items you have so far and the idea of wallpapering the back of the shelves.  I would love to add some colorful old books to stack and fill the space, as well as some more bright + colorful accessories.  We can also choose something coordinating for the mantle on each side of the television.

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