First issue is the GIF format (only 256 colors). Don't use that format.
To add a "black" box (or any other color) to your existing QuickTime file you should first make your image file in a format (other than GIF) that also supports transparency.
Depending on your image editing software the PNG format is the best choice. But JPEG will also work if your use 32 bit color (no transparency layer).
To add this image to your existing QuickTime file follow these steps:
Create the image file (1 bit for just black and white colors) and size it to the dimensions desired. JPEG or PNG will work nearly the same but JPEG doesn't support alpha channels.
So, lets say your have a 320X240 QuickTime file but want a "box" around it sized at 320X320 (square).
Create a 320X320 all black image saved as PNG or JPEG.
Open that file with QuickTime Pro, select "all" (Control-A) and "copy'"(Control-C).
Switch to your 320X240 QuickTime "video" file.
Select all (Control-A) and move to the Edit menu. Pick "Add to Selection & Scale". This will add your image to the entire "video" portion.
Control-J to open the Movie Properties window.
Highlight (single click) on the "Video" track portion of your new composite video.
Click the "Visual Settings" tab.
Here is where you change to position of your video track in relationship to your image track.
The "Offset" is what you need to adjust (because your video is smaller in dimensions than the image track).
Default positioning is 0,0 (upper left). You need to add new values to the offset to set the position of the video track. It may take a few tries to get what you want. Changing the offset values until you get the size and shape you desire.
Save As. Give the file a new name and save as "self-contained" to make a new file.
These same steps can add "video backgrounds" to another "video" track.
One of my pages to show a QuickTime example:
http://homepage.mac.com/kkirkster/crosstown/index.html
