Pictures have a special place in all video editing. They can work for or against you depending on the rhythm and purpose of your work, but all editing applications must be compelled to allow them. Each program addresses them in a different way, and Windows Movie Maker has its own unique format.
If you want to import a still picture then there are a few different ways to do it. You can go to File in the task bar and then down to Import Into Collections, which is also Control and I as a keyboard shortcut. This allows you to bring in any kind of media file into your Collections folder, pictures included. You can also go into the Movie Tasks bar on the left hand side, look under the Capture Video setting, and select Import Pictures.
Once you do it will be put into Collections right along side your video, unless you select to put it into a special folder. This is probably a good idea since non-linear video editing is essentially media management and organization is key. When you drag and drop the photo into your Timeline it is automatically going to exist as a five second video clip. This can be lengthened or shortened depending on preference, but this needs to be done in the Timeline.
Most video editing applications allow you to do fundamental changes to the shape and motion of all media clips with a special premium being put on pictures. This is done in order to allow you to conform your photo to different aspect ratios, add photo motion for energy and focus, and even enlarge images to bring attention to specific areas. In Windows Movie Maker you are reduced to using only the video effects that are standard for all clips but in no way curtailed toward photos. This means that you will end up having the ability to add filters for desaturation and artistic color changes, but not the standard photo effects that you usually see in film.
Over all photo use should probably be fairly limited, but you can easily create a photo slideshow in Windows Movie Maker. To do this you simply line up a number of photos against each other in the Timeline, even adding cross-fade transitions between them. You can add music to the audio track to accompany the photos if you want. Thankfully Windows Movie Maker already interprets photos as five second clips so you will not even have to change the length of the photos for the slideshow.