Put border around partially transparent Image being drawn on CGContext
One way to do this is to use the mathematical morphology operator of dilation to "grow" the alpha channel of the image outward, then use the resulting grayscale image as a mask to simulate a stroke. By filling the dilated mask, then drawing the main image on top, you get the effect of a stroke. I've created a demo showing this effect, available on Github here: https://github.com/warrenm/Morphology (all source is MIT licensed, should it prove useful to you).
And here's a screenshot of it in action:
Note that this is staggeringly slow (dilation requires iteration of a kernel over every pixel), so you should pick a stroke width and precompute the mask image for each of your source images in advance.
I would try either setting the stroke color and line width before your call to CGContextDrawImage, or tweaking the shadow (opacity, blur, etc) so that it looks like a stroke around the image. Let me know if this works!