I'm not sure just what you mean... A .MVP project is a bit like a recipie -- use these ingredients (video clips, images, music), follow these instructions (order of things, cut or fades etc), and perhaps some decorations (special effects). In this sense the project isn't ever "finished" as you can always make changes and create (export) a new version of your video.
To add a picture you would at its simplest move everything beyond the insertion point further along the timeline, place the new picture and then close the gap. Things will be more complicated if you have a separate audio tracks which would have to be adjusted for the increased movie length.
Assuming you have already got a finished video (.mpg, .mp4 etc) an alternative would be to start a new .MVP project with your movie as the only object. Then you can make a single cut, move the tail section as above, and insert your new photo.
Plus titles, PiP's, collages, etc. It's best to learn how to move things safely (or not) by watching my tutorial. At least I hope that I explained this in great detail. Another way, shown in the tutorial, is to place the playback marker at the insert point, select the clip to insert and press on 2 on the top row of keys (not the nums pad). This inserts the clip/photo and moves everything to the right, but as you pointed out, can play havoc with things on other tracks.