Easy-peasy! Click on your nickname at the top of the page (towards the right hand side to bring up the handy drop down menu) and then on "Upload Pictures".
Now click on "Choose File", find the picture that you want to upload on your computer, select it, and hit the appropriate button in the file dialog - depending on your browser and language that might be "Save", "Open", "Grzblng", ... the one that doesn't say "Cancel".
You can enter a caption as well if you want to, and you can make sure that only your friends see the picture by clicking on the "change" link in the "Privacy" line and selecting "Just Friends" from the dropdown list, or if you want this picture to be your main profile picture, check that "Avatar" checkbox. However, an "Avatar" cannot be set to "Just Friends".
Now hit that "Upload Picture" button, have a bit of patience, and you'll get redirected to your newly uploaded picture. Purty, innit?