Take a look at the very first answer in Macro to Combine Arbitrarily Sized Images in which I recommend using Imagemagick (a separate install) and its convert command in an Execute Shell Script action:
convert ss-1.png spacer.png ss-2.png +append ss-combined.png
That builds the combined image horizontally. To build it vertically without a spacer:
convert ss-1.png ss-2.png -append ss-combined.png
Simple as that. If you install Imagemagick, that is.