Quick Thumbnail is really the fastest and the most powerful way to resize images online (atleast as far as i know). They have a very clean, less-confusing interface. they provide various options to resize the images :

1) Resize by Percentage
A slider is used for this that ranges from 0% to 150%. Its the best bet to resize a photo without any distortion.

2) Resize by Fixed Sizes
They have six sizes listed and the good thing is that you can select even all the six.

3) Resize by Standard Sizes
A very long list of standard sizes ranging from 16 x 16px (small icon) to 1600 x 1200px (21 inch monitor).

You can use all the options at create multiple resized images, which makes it even easier.