Download a copy of IrfanView for resizing. I find that to post a picture here about 700 pixels wide is as wide as you want to go. This way the viewer doesn't have to scroll across to see the entire picture.
As far as formats? jpegs will work here. Make sure you have typed the correct extention. Sometimes (most times) jpegs have a .jpg extention. You need to type everything exact.
Make sure you have your vB capability code ON, as well as your HTML code.