To use a picture, either in your profile or in a post, it must be hosted on the Internet.
You can steal the url address for any picture on the Internet by simply right-clicking on it and choosing properties. Web sites prefer you don't steal their pictures, though, and it could slow down their server, so it's better to save the photo and upload it yourself.
There are many picture hosting websites, but I use www.imageshack.us. You don't have to register to use it.
Once the picture is on your harddrive, go to imageshack and browse for it. Then click 'upload' and after a few seconds it will generate the code for your picture.
To display the picture in a post, click the button, then insert the url address between the tags.