A flatend png is normally smaller than a jpg, but not out racking a tiff by a long shot. For ezness i would suggest using a jpg format, when uploading a screenshot to quad, and also when putting in the mapshot (the only reason being that all the map, for the most part, have jpg anyways). The other two formats are not worth your time, i would never use either one.
To convert a png to a jpg use gimp (listed above), or photoshop (if you have it). Try to aviod useing paint for reason i can explain if you want. Simply open the shot in gimp or photoshop, click saveas, then in filetype select jpg (or whatever formate). Then click save.
If you have any questions look at gimp/photoshop help, or ask on here. :)