You should make sure you uploaded all the images to your
webserver as well as the index.htm page. If you post a link to the
problem page it also makes it easier for people to help you out.
I don't think the valid editor message is related to your
problem. It could be that you don't have a program on your computer
that can edit your image files.
Also, FYI, if you have more problems like this, they're
probably better for the Dreamweaver General Discussions forums. The
extension in the error message refers to your images file extension
(.jpg, .gif, ...) while the Extension in the forum title refers to
more topics about extending dreamweaver and Dreamweaver Extensions
(aka plugins).
Hope this help,
Chris
Adobe Dreamweaver Engineering