Currently the background sets within distributions (tend to) contain multiple copies of the same image in different aspect ratios. What could be done is to allow cropping of one image to a desired aspect ratio reather than distributing all possible combinations as individual images.

So currently we have:
<from>
    <size width="2048" height="1536">4-3.png</size>
    <size width="1920" height="1200">16-10.png</size>
    <size width="1600" height="1280">5-4.png</size>
    <size width="3200" height="1200">8-3.png</size>   
</from>

And say we are interested in the stuff in the bottom right corner we add an X/Y offset:

<from>
    <size width="1600" height="1200" x="1600" y="0">8-3.png</size>
    <size width="1920" height="1200" x="1280" y="0">8-3.png</size>
    <size width="1500" height="1200" x="1700" y="0">8-3.png</size>
    <size width="3200" height="1200"               >8-3.png</size>
</from>

Patch is attached.