The full version of Vegas has unlimited tracks, so it's easy to have dozens or even hundreds of images in the frame all at once. Even 4 tracks isn't a horrible limitation. You can create a sub-project with 4 images at once, render it to a new DV .avi file, then use this new file in a project to have four images at once on a single track. Rinse, repeat ... and you can build up projects that look like they used dozens of tracks while only having access to 4.

Check out keyframes in the manual or help screen. These will let you change the track motion parameters from point to point on the timeline as often as you wish. After you're done with an image, you can change the track motion size and position back to normal, or to whatever you want for the next image that appears on that track.

By the way, i'm a tad bit confused by your statement "why does the track go black after the media event?" This is what is supposed to happen, whether you use track motion or not. If there's nothing in the track, Vegas shows black. Unless, of course, you have something else on a lower track, then this will show through wherever you don't have something on the higher tracks. What's actually happening is that when there isn't any media on a track, Vegas shows whatever is behind that track. If there is media on lower tracks (those with higher numbers, farther down the screen) you'll see that media. If not, you'll see the same black background that you normally see before putting anything in your project at all.