It's interesting to see the dimensions of images compared to their size in KB.
Image by Pabitra - size 102 kb.
Image by chrisild - size 42 kb.
Screen capture by <k> - size 118 kb.
I still use PaintShopPro 2000. Generally I use the "compress" function. For a small image, I compress by around 18%, for medium images from around 24% to around 32%, but around 40% or more for very large images. Normally you can't tell the quality difference if you use the compression function wisely. It would be interesting to hear about your methods. Perhaps not all graphics packages include compression functions these days, but that leads to a lot of wasted space when small-sized images get uploaded with heavy file sizes.