Switching from PNG to JPEG
You might also want to convert iWeb’s PNG files back to JPEGs, though this isn’t as important as the preceding tasks. You can use any number of image editing tools to do convert the files and you can even automate the process (either by using an application that includes that feature or Mac OS X’s Automator utility). JPEGs are typically preferred by Web designers because they have a small file size footprint and thus download faster. If you keep the filenames the same, you can accomplish the update to the site by using a single global search and replace.