$inputwidth = 100;
$inputheight = 100;

// So then if the image is wider rather than taller, set the width and figure out the height
if (($avatar_width/$avatar_height) > ($inputwidth/$inputheight)) {
 $outputwidth = $inputwidth;
 $outputheight = ($inputwidth * $avatar_height)/ $avatar_width;
}
// And if the image is taller rather than wider, then set the height and figure out the width
elseif (($avatar_width/$avatar_height) < ($inputwidth/$inputheight)) {
 $outputwidth = ($inputheight * $avatar_width)/ $avatar_height;
 $outputheight = $inputheight;
}
// And because it is entirely possible that the image could be the exact same size/aspect ratio of the desired area, so we have that covered as well
elseif (($avatar_width/$avatar_height) == ($inputwidth/$inputheight)) {
 $outputwidth = $inputwidth;
 $outputheight = $inputheight;
}

return '<img src="' . (str_replace(' ', '%20', $avatar_img)) . '" width="' . $outputwidth . '" height="' . $outputheight . '" alt="' . ((!empty($user->lang[$alt])) ? $user->lang[$alt] : $alt) . '" />';