Need we say more?

http://pixlr.com/editor/

If you’re uploading images that are humongous, it’s going to take forever.   If you open them up in this editor, you can visit “Image” dropdown, where you’ll see the image size and you can make the image smaller – we recommend about 1000px wide maximum to upload for your site.  Typically the main column of content on your website is between 500 to 700 px.