HI --- pictures displayed stepped up the stairs are too hard to view . I think if photos can be displayed in groupings at even levels like a group at the bottom , another grouping on the landing and then another at the top landing . Much easier to enjoy when standing on a level area.I think it looks beter too. I find that sometimes these areas can get too cluttered , so plan carefully. Plan you groupings on the floor until it is pleasing to your eye , then transfer to the wall. There are placement patterns available at frame and large hobby shops etc. that can help.

Perhaps a large quilt or rug would work for you. But if you want photos use moulding to make a large frame going from three feet below the ceiling to three feet (approximately) above the landing floor. Take similarly framed photos (like simple black frames) and arrange geometrically inside the frame.

I think the biggest problem is that the ceiling height is different near the landing...I don't know how to fill both spaces without it looking funny.

I have a large metal tree I would like to use as well.

I don't know what your color scheme is, or if you have kids, but I think a fun wallpaper would be great right there: [houzz=] [houzz=] [houzz=] [houzz=] [houzz=]

This might help with your dilemma. The moulding would be shaped to fit your area.
http://www.hgtv.com/design-star/design-star-how-tos-clever-diy-projects-inspired-by-the-show/pictures/page-3.html

I love the quote with photos idea. I have sold several of these decals to customers and they were all very pleased with the results. The quote is a design from Uppercase Living. This expression (vinyl decal) comes in 3 sizes & over 50 colors (item # 11513,11514,11515) and is available on my website: http://jeand.uppercaseliving.net" (Photo credit: thriftydecorchick)
Direct Link to this item pictured below: https://jeand.uppercaseliving.net/DesignItems.m?DesignId=742&MenuLinkKey=products&CategoryId=238

Another idea that I like.Traditional Staircase design by Portland Design-build Angela Todd Designs
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If you have a large metal tree, I would hang it in a central location and then make paper templates of the photos you want to hang, place them on your wall with tape and move them around until you have a pleasing set. I did "pin" to one of my pinterest boards something someone had posted of different ways to place multiple pictures on a wall. I am Nancy Hehmann on pinterest.

If you are interested in vinyl decals of inspirational sayings, go to Etsy. com and input that in their search engine. They also have photography, paintings and other wall hangings as well as lots of home decor, jewelry. All unique and created by artisans all over US , Canada etc. I have a photography shop there, nancyhehmann.

Have fun!

Lefty - right on! Always create on floor - let it sit there for an hour or two and make changes as you pass - incorporate the tree as well - I do agree with the above - add textures - metal pieces and frames and sayings - - it creates an interesting focal - know you will love an eclectic motif!

Some product ideas:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120991938227&var=420095656385

http://www.ebay.com/itm/20Pcs-Wood-Picture-Photos-Creative-Combination-Wall-Mounted-Set-Photo-Frame-QT11-/280973260785?pt=Photo_Frames_Display&hash=item416b4fb7f1

http://www.ebay.com/itm/17-PCS-reality-wood-PICTURE-PHOTO-FRAME-WALL-SET-ART-work-HOME-DECOR-mars10-/261030880791?forcev4exp=true&forceRpt=true