Usually your hallway will have two sides, select the one side of the hallway for your military pictures. Consider this wall for not just military photos, but photos of building America or ther accomplishments. This way you have an opportunity to present photos of Pony Express riders, U. S. Marshals, Undersheriff, or other government positions in your heritage.
Start at the top of the wall with ancestors of the American Revolution, for example, or the earliest war of which you have record. If you have a picture of your relative during the American Revolution, you have a priceless treasure, but most of us do not. One thing to do is find out the best you can to see if your relative was a fifer, drummer, or a soldier, or maybe served in the Navy. This will give you a good start. The Daughters of the American Revolution will lookup your ancestor if you have a name and what state he lived in during this time. Here is the web site .
After finding out what type of involvement your ancestor had during the American Revolution, there are several sites on the web which give good colored sketches of the uniforms, weapons used, and some sites even show indicate the pay. Use GOOGLE to aid you in finding what you are seek.
Find a colored sketch of thetype of uniform your ancestor wore and print it. Place your ancestor's name below the sketch and frame it. This will put a good image in your mind of what he wore during that time period.
For the War of 1812 and French and Indian War, you can find similar examples. When you start working on your genealogy for the Civil War period, this may become a little easier, as there are more complete records kept during this time. It's possible to even find complete Unit photos on the Net. These all make wonderful memories. If you have a picture of your Civil War grandfather in civilian clothes, take a close look at his lapel. Oftentimes, these brave men who fought in the Civil War were very proud of their service to our country and wore their medal pinned to their lapel in photos.
As far as WW I and WW II and other wars forward, I would check with other family members to see if someone might have photos of them in uniform. If not, check on the site,  and look for the discussion groups for WW I, WW II, or the more recent wars to see if something about your ancestor is mentioned.
For other government positions such as Pony Express Rider or U. S. Marshal, take a tour through your local antique store to see if you might be able to find a badge relating to their service to our Country. Ebay is also a good place to check for such memorabilia. When you find your treasure, polish it well, and mount it on a piece of felt--green or white usually works well. Below it put your ancestors name. This can make a wonderful memory of those in your family.
Now it is time to organize your photos to place on the wall. Arrange them according to dates, and start at the top and work down. Once completed you will find you have quite a collection and something to be proud of on your wall.