languages/english/editbasic.php

Find on line 30
Code
$ubbt_lang['DIMENSIONS'] = "(Your image must be smaller than %%MAX_X%%x%%MAX_Y%% pixels; .gif, .jpg, .png accepted).";

change to

Code
$ubbt_lang['DIMENSIONS'] = "(Your image must be smaller than %%MAX_X%%x%%MAX_Y%% pixels; .gif, .jpg, .png accepted).
<br />
<br />
To resize your avatar to fit our avatar size, please click <a href='http://cranno.com/?w=80&h=80' target='_blank'>Here</a>
<br />";

My site avatar is 80X80 so you can change that in the url