Resizer is the online resize tool for all your jpg, png en gif images. The great thing about this is that everything is online! This means you don't need software that you have to install on your computer.
The usage is simple: You chose an image, you may already the new dimensions and click on Resize. Then, you can play with the slide bar to get the perfect dimensions. When you have the right dimensions, you can press "download" to download the image.