Change to different uploaded version/size

Hi,
I have the following problem: 2 out-of-10 Thumbs (150×150) that are loaded in the catalog are gone. Removed by accident.

Now there are larger images generated and available. Like 200×200 and bigger.

I want to load these bigger versions if the 15×150 version does not excist.

How can I archieve this?

I am already working with some code like this:

function woocommerce_template_loop_product_thumbnail()

		echo woocommerce_get_product_thumbnail();

}

Just this code does not say: IF the 150×150 size is not available take the bigger version…

How can I archieve this?

I would reaaaaly like it if someone can help me with this code!

Thanx in advance.