[Maddox Bestpage]

Here’s an updated version of this that works in CC as of 2016 (tested this with a transparent PNG file with a complex transparency shape, seems to work fine):

Add: Effects -> Paint Bucket, then use the following settings:
Fill Selector: [opacity]
Tolerance: [60 – adjust until appropriate]
Stroke: [Stroke]
Stroke Width: [15 – whatever you want]
Color: [whatever you want]
Opacity: [100%]
Blending Mode: [Normal]

Caveat: make sure the image safe area (any area you want stroked) is completely enveloped in the transparent area, because the transparency won’t apply to the border of the actual PNG/video file (confirmed this works with MOV files with exported alpha), just the transparent pixels. It’s easy to add a transparent border to any image.

Note: if you find it stroking pixels inside your image that you don’t want, just turn up the tolerance up to 255. It’ll stick to the outside of the image rather than “holes” you may have inside.