I like to set the width of “wide” pictures to 500 pixels. I set the height of “tall” pictures to 700 - this comfortably maximizes the size of the pic for a 600 x 800 display.

Then the image needs to be compressed in order to ensure that it renders promptly, and is efficiently stored on the web server. Here's how:

iPhoto

For all you mac users out there, you can simply select the photo(s) you want to use, and then click Share→Export. Enter a width value of 500, and follow the instructions to finish exporting.

Adobe Photoshop

Open the image and rotate it as necessary
Resize the image
- Click Image→Image Size…
- Enter a width of 500 pixels, the height should change automatically.
- Click OK
Save for Web
- File→Save for web…
- Choose JPEG and Maximum from the dropdown options.
- Click OK and save the new image in a place where you can easily retrieve it for uploading.

GNU Image Manipulation Program

If you can't get your hands on a copy of photoshop, this software can stand-in for most uses and it's free… http://gimp-win.sourceforge.net/stable.html

Open the image and rotate it as necessary
Resize the Image
- Image→Scale Image…
- Enter a width of 500 pixels
- Click Scale, the height should change automatically.
Save the Image
- File→Save As…
- Rename the file appropriately
- Expand Select File Type (By Extension)
- Select JPEG image
- Click Save
- Check the box Show Preview in image window
- Move the slider to the desired quality (I like 95 or better)
- Click OK

Irfanview

Here's another one that should be pretty simple to setup and it's free to download and use. I have not tried resizing/optimizing for the web with this one but it should be pretty easy. Directions forthcoming… http://www.irfanview.com/