Click the browse your computer button and navigate to your image, then click Start Upload
After the upload completes you will see this dialog. On the right side, in the window titled Share this Image are several sharing options. Click in the box titled Direct Link and copy that string (Ctrl-C)
Paste into the body of your message or the URL line in the gallery.
Caution! Imgur will not resize your image, so it may appear much too large! You can either resize it before uploading it, or use the Edit image tools after it is uploaded. If you choose to use the editing tools on Imgur, click the Crop/Resize button and edit the largest dimension; 800 is a good choice (640 and 1024 are other common sizes).