I'm another Irfanview user. I have various other tools, either from the web, or that came with my camera(s), but Irfanview is a nice combination of viewer, and manipulator.
Keep in mind that there are two things that you can control - the size of the image (in pixels), and the compression (for jpeg images). With both you are going to loose detail - you can't get away from that when you reduce a 2 to 4 megabite file to 100k or less for posting.
Perhaps the best way of minimizing the loss of detail while reducing the file size, is to crop - posting just the part of the image that matters, while cutting out most of the background.
paulj