Remember, this is a web site, not Photoshop. While it will resize the images to the correct size you want, quality may drastically decrease the further you get from the original size. Always keep backups of your original images!
To use the resizer tool, upload your image to the right. You have two options when resizing your image.
Resize by pixel allows you to enter custom numbers and will not maintain the aspect ratio.
Resize by percentage will increase/decrease the image size based on the percentage and will maintain the aspect ratio of the image.