I would suggest you to consider other available options before proceeding to resizing the image. That's because image resizing will always downgrade the quality of your image. If you are going to use only a portion of the image, I would suggest you to crop the image a day use it.
Having said that, here are a few available methods for resizing the images -
1. Online tools
If you do not have any software installed, or do not want to install any of them, you can opt for the online tools available for the purpose. Just upload the image to the website, set the size you want to resize it to , follow on screen instructions and you will get the resized image ready for download. Some such sites are http://www.resize2mail.com, http://www.shrinkpictures.com and http://resizepic.com.
2. Using Paint
Paint is the most powerful image editing software that comes with Microsoft Windows. Open the image in Paint. Go to Home tab,then to Image group and then to Resize . You will two options to reduce image size - by pixels and by percentage. You can choose either of them, but make sure you have kept Maintain Aspect Ratio option is checked. Once done, save your resized image by using Save As option.
Open your image in Picasa. Click on Export button at the bottom. In the Export to window, slide the Resize to bar to the size you wish to.
These are a few methods to resizing the images. There are many such options available over the Internet.
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