On your profile, there should be something called "Edit Profile". Click that. Once your on that page, find an image (in a new tab) to fit into the URL area. It only can be 100x100 though. If you find an image you like but not 100x100, Paarthurnax can resize it for you!

Okay i will do this step by step

1. Make sure your logged on

2. Go to your profile

3. Click "Edit Profile

4. Below your profile picture there is a bar

5. Open a new tab

6. Search for an image you want

7. In google, go to search tools

8. Click "size"

9. Click "Exactly..."

10. In the width box, put 100

11. In the height box, put 100 (the reason for this is because profile pics must be 100x100 or smaller)

12. Click "Go"

13. Find the image you want

14. Copy URL, or in other words the link. (Do this by right clicking, then go to copy image location)

15. Paste the URL in the bar under your current profile picture

16. Almost done! Now click "Save Changes" at the bottom.

17. You now have a new profile picture

Does this help?

Note- 1teps 1-3 is also how you edit your Bio, just click the "Bio" Box and type what you want in your bio!

Find a picture on the Internet and copy its URL onto your edit page. 

See mirkrilaar's answer for a better answer