You really don't need any extra programs....
The AH screenshot format is a BMP; which is "big" (file size) format.
However, if you open it up in good old MS paint, you can save it as a jpeg (choose "save as", then jpeg) and the file size will be considerably reduced.
Paint will also resize the image, if it's needed, to bring it in line with size limitations.
So, to recap - additional programs are nice for re touching or editing images - but if you just want to post here, a standard windows installation is enough.