How do I reduce gallery storage?

Hi,

Due to a computer glitch I have way too much storage in my gallery (1020 MB too much, we are using 7% of it) as you can imagine its jacking the cost of buying time credits through the roof. How do I reduce the gallery storage size? I cant find any topics explaining how nor can I find where to do it on the forum

Thanks in Advance for any help

If you already spent the credits, now no way to reduce the size.