.mobi file questions

I have a template for doing a fixed layout epub, the template has 5 pages
xhtml
css
a folder for images
content.opf file

I have the actual .mobi from these and I want to use different images and have to ask the following.

I'm adding the new images thru Dreamweaver and once I add all how do I save the files and create a new .mobi out of them.

For instance, I can open an epub in Sigil, edit html and text, save it as an epub. But Sigil will not open a .mobi file.

RD