to be wide or not to be wide

As you can see, the pictures are different ratios today. One is in the traditional 4:3 ratio (the top one) and the other is in 16:10 widescreen, a favored ratio for widescreen computer displays.

I have to credit the user Express at this thread on Neowin Forums for this stuff, although I changed the formula around to convert from widescreen to 4:3.

So, here we go:

Converting 4:3 to 16:10 widescreen

your widescreen screen height (for instance, mine is 800 pixels) times the width of the 4:3 photo
now divide that number (the product) by your widescreen screen width (mine is 1280 pixels)
this number is what the new height of the photo needs to be to be to correct aspect ratio.
The widescreen ratio we're talking about is 16:10, or 1.6. Fire up your favorite editor that will allow you to do this precisely (I use the GIMP for this) and crop the height to the number you got. Your editor may show you the ratio you're getting as you crop it. However, I advise using this as a second reference and going for precise pixel values.

Converting widescreen to 4:3

I did this with a 16:9 widescreen photo my camera took (Dworshak Dock), but I think it would work for 16:10 images too.

your 4:3 screen height (for instance 1024 pixels) times the width of the widescreen photo
now divide that number (the product) by your 4:3 screen width (say 768 pixels)
this number is what the new height of the photo needs to be to be to correct aspect ratio.

Feel free to ask questions.