My question might have been a little confusing, so let me clear it all up:
I have installed the filesystem for my RPi on a USB stick to preempt the corruption of the data on my SD card. That's all fine and dandy, and it seems to be working. Unfortunately, my SD card is now far too big for its breeches.
I used an 8 GB card, mainly because it was all I had. So, I'd like to find a way to use all that space. My idea is that I could simply use the SD card to save camera images on, once my Pi camera gets here. Since SD cards are essentially designed for this type of use, I figure it is a good way to avoid corrupting anything, and I will be able to use all that extra space that would otherwise go to waste.
To conclude, I'll bold a tl;dr:
Can I use the SD card to store rarely-used files while using the USB filesystem that I already have in place for everything else?