Before we start hacking away at our image with the Slice tool, we should discuss a few issues worthy of note (and perhaps a bit of soapboxing for good measure). First, why are we slicing in Photoshop instead of ImageReady? Simple. Because we can.
Photoshop is not as limited as ImageReady. It has more and better filters and the full contingency of bitmap editing features, whereas ImageReady is stripped down. So, if we stay in Photoshop, we're less likely to have to bounce back and forth if we need to do a Gaussian blur or something.
Slicing using which program? Aside from the aforementioned issues, there's really no good reason to slice in Photoshop rather than ImageReady. The fact is that which one you use is a matter ...