If you just want to quickly resize an image without having to install a program, there a numerous websites that you can visit. Resizr.com is one of the easiest websites to perform a resizing task. You can resize an avatar, photo, picture, logo, and more..
It is so easy to use, you just have to slide a bar back and forth until you find your desired image size. It usually takes less than 60 seconds to upload the image, resize it, and then save to your computer in an image format of your choice. As you are sliding the marker across you can see the details of your image change in the boxes below.

You can also adjust your image, rotate it, change it to sepia, black and white, crop it, and more.

1. Simply select an image from your computer by pressing browse or type in  URL of an image.
2. Then press on Upload image.
3. Now you will see a slider bar with small and big at each end. Slide the marker to resize your image.
4. Below you can see exact size details.
5. When you are finished click on resize my image button.
6. Now all you have to do is right click on the finished image and choose Save picture as.

I seriously suggest you bookmark this website as it is an extremely useful online tools.

Go to the Resizr homepage