When you need to resize and image use the Social Image Resizer Tool.
Source: Social Image Resizer Tool
So, you took a picture with your smartphone and would like to use it as your Twitter avatar, or post it to the web. The problem is that Twitter will yell at you for uploading such a large picture, and the internet will yell at you for making them download such a large image. This is where the Social Image Resizer Tool comes in.
To get started, click the Choose File button to select the picture, and then press upload.
From there the controls are limited. Clicking and dragging over the image will crop. Under the Choose a Size dropdown are several preset sizes for various services, or a user input option that lets you enter the size. In the example above I entered a custom size of 640×480.
The Format dropdown lets you select one of the following output formats: jpg, png, gif, or ico.
When done, click the Done Editing button and either download or send the picture over email. Very quick and easy, without requiring a log in!