My screenshots come out as 1280 x 960 Pixels (1.23 MPixels) (4:3), I believe its set to the resolution that I'm using, don't quote me on that though.
I load the image into Irfanview and go to the IMAGE menu and select RESIZE\RESAMPLE, change the width from 1280 to 800 and save it as a JPG, if you need to crop part of the said image out select the part that you want with the mouse and then go to the EDIT menu then select CROP SELECTION, save as a JPG and job is done.