FOUND IT!!!
Line 152 of catalog/includes/modules/better_together_promo.php
Change
return tep_image(DIR_WS_IMAGES . $product['products_image'], tep_get_products_name($product_id), $width, $height);
To:
return tep_image(DIR_WS_IMAGES . $product['products_image'], tep_get_products_name($product_id), x, x);
x being the number you want to set it to.