That little picture under your name is called an "avatar". Click on your won name and that will take you to your profile. In the left margin you will have a list of many choices....choose "avatar". And then that will bring up the avatar page. Then you will have three choices......"choose avatar"...."upload avatar"..."link to avatar". The firwst one gives yo the ability to choose a "canned" avatar. The second one allows you to upload a pic off of your computer(which is the one you are looking for I think). The third one would allow you to link a pic off of another internet site.
So click on "upload avatar". When yo do, you will see a "browse" button under the topic "upload images". Click "browse" and what you will be doing is browsing your own hard drive and choosing a pic. So it would be a good idea to locate the pic you want and know where it is by the time you get to this point. You then double click on the image you want and then click "upload image". You will notice after a few seconds that the image will show up in the current window. You have to actually select the image(when you do a blue square will show up around the image. And then click the bar that says "select avatar. And that should do it.
Tip: This picture is going to be small anyway so there is no need to upload a high res pic. The program will take it but the larger the pic, the longer it will take to upload(minutes for a big one....seconds for a small one). It would behoove you to resize the pic before uploading.