Right Adele, If youv'e got software on your computer for resizing photos e.g adobe photo shop.
Press browse in your photo software, You then press edit it will give different options, one of these is resize image. You then type in the size you want the picture to be (70x70 pixels for an avater), The picture will be resize, press the save button and save it in your documents.
Next come onto this site press quick links, Then edit avtar. At the bottom of this page is an option to add your photo, Press the browse button and select the photo that you have resized. Press save changes at the bottom of the page. Fingers crossed your photo should appear.
Hths