Ta add a stroke to a raster image (other than work around it by using the previous solution):
1. Select the image;
2. In the appearance palette (the greatest thing since sliced bread, learn how to use it quickly), click on the flyout arrow and select "add new stroke"; Nothing appears to happen, yet, so next:
3. Effect>Path>Outline Object will make the stroke visible. Now, you can access this stroke from the appearance palette and change its color, width, and a whole bunch of other things.
The benefit of doing it this way, is that you only end up with one object, so no worries about scaling or moving and overlooking the stroke in the process.
If you need the stroke to be a separate path, Object>Expand will make it "real".
If your raster has transparency, the path will follow the edge of the visible pixels. That can be nice, too.
Bert
Bert Philippus