Assuming your PGraphics instance is called 'pg' just use 

Ok... well, in the sketch where I use this with a PGraphics (see below), I only save on a mouse click, and have a counter to iterate saves, so I'm expressly setting the name... which is why I get an error... responded quickly and didn't think about why I had one and not the other...

It is better in your case, as you have consecutive numbers. Perhaps better is to use nf(saveCount, 3) (for example) to have numbers aligned and properly ordered (although modern file managers are able to sort correctly such files now).

thanks, save works, and nf makes it complete.