only allow to upload profile pictures of a certain size
You need to form alter user_account_form().  Remove user_validate_picture() as the validator.  Duplicate that function into your own module, and add as a validator.  Inside that, update the size line to be:

'file_validate_image_resolution' =&gt; array('160x160', '160x160'),


This should enforce the same value for both minimum and maximum size before the file gets saved off.  I don't think you can add a second validator; I think the size check needs to be done in one step.