First, you'll need to upload your picture to an online image hosting site. I usually use http://postimage.org/ . Other options include, but are not limited to, Photobucket, Imageshack and Tinypic.
Using Postimage for this example:
Next, you'll see the following screen:
Done! This will create a small thumbnail like below. Then someone can click on it to enlarge it to view the full size image. This helps pages load faster for everyone by frontloading smaller images yet still having access to full resolution for greater detail.