My apologies for being so straight-forward, but you can use freeware program IrfanView (small executable file) for resizing your photos. On forums people generally ask to keep photos down to 500x500 pixels, but sometimes picture is worth 700 pixels.
In the same program you can make the photo lighter, more contrast and sharp, correct color shades. Very simple and efficient, unlike big programs.
Another option is using Webshots (free too), it places thumbnails on the page, who is interested - may click on them.