If you are using Windows, the "easy" way is to open the pic in Paint. Select Resize, then click the Pixel radio button and enter something reasonable. Save the pic, done. Usually for a landscape 1024x? and for portrait maybe 600 or 800x ?
Another trick for Windows users to grab something off your screen, browser window, etc - look for the Snipping Tool. It is used to capture anything off your screen and save to a JPG file. You can also mark up the capture with colored markers. The attached pic was captured with the Snipping Tool.
I personally use and semi-old copy of Fireworks to modify and enhance pictures, but I've been doing that a long time. Non-trivial type operations. I also own a big chunk of web space and hosting at GoDaddy.