From CreativePro.com –Add a Stroke to a Placed Image in Adobe Illustrator:
Once you’ve placed a photo image in Illustrator it would seem to make sense that you could just add a stroke around the outside in the same way you do in InDesign. After all, you’d imagine that the image is inside a rectangular frame, and it’s just a matter of choosing stroke color and width, right? Well, if you’re thinking this way – you’d be wrong. Unlike InDesign, when a photographic image is placed in Illustrator, a frame is not automatically created to contain it – the image is just hanging there, kind of loose and uncontained. So, if you want to add a stroke to the image (or a colored tint for that matter), you’ll need to manually add the frame first. You could just draw a rectangular frame around the image, and add the stroke to that, but this would create problems when you want to resize or reposition the image and you discover the frame doesn’t move along with the image. The quickest solution is to select the image and click the Mask button in the Control panel. Then you can apply a stroke and adjust it just as you would in InDesign.