I'm not exactly sure what you mean here, but let me have a go at explaining what I THINK you mean.
If you make an image in fireworks, and save it as .png you can come back to it at a later date and change it's components like text, colour etc.
Or, if you wanted to cut something out then you could use the marquee tool. If neither of these are what you mean then give an example and I'll try to help some more
