I'm guessing you don't have any top-of-the-range graphics software installed, so I'll tell you how it's done on paint.
Open up your desired avatar picture in paint.
Select it using the select tool.
Press ctrl + e.
In the bixes 'Width' and 'Height', type 75 into both.
Check the box that says 'pixels'.
When you're done, click OK.
Then save your image.