It looks like the new photo filters in iOS 7 are lossless. That is, the original, untouched image file is saved as is, and the effect is applied on-the-fly. This is how non-destructive editing apps like Lightroom and Aperture do their thing.
What does this mean for users? It means that you can shoot away in black and white and return to the original colored pictures later. In theory, it’s a lot like the existing image editing tools in iOS, which let you apply crops and “enhancements” and then remove them at any time in the future.
Right now, it also means that your filtered photos will who up unfiltered in other places. In fact, I firs discovered this when I noted some cool, dramatic B&W photos I’d snapped on my iPhone 5 were showing up as insipid color snaps in my Photo Stream. The good news is that exporting the pictures keeps the edits (I opened an image in Instagram on my iPhone running iOS 7 and it was in B&W).
I also noticed that – when I beamed a bunch of photos to my Mac form my iPhone using PhotoSync – an XML “sidecar” file went with one of the pictures. I’m pretty sure I’ve seen this before in transfers, but now it – theoretically again – means that the filters applied to the photos on iOS could be reversed and tweaked by iPhoto on the Mac.