The link that best explained to me how to control your cache was mnot.net. I don't think messing with the default-master would do much.
You should run a filter from your tcl/init.tcl file. If you don't want or care about controlling access to your images, just have the filter also return the file and skip openacs completely. You still need to also skip out of any trace filters.
Another option that takes care of the few details of programming is to use my VAT module. You would need an additional domain, or subdomain name for the image files in that case. However, I would guess configuration of my module would take about the same amount of time as writing the filter.
If you do change the domain name for the images, you could use a simple static webserver for them as well: thttpd or publicfile might work for you.
