could it be that you are working with a German DW-version of DW? (Cause of the need to replace the umlauts.)
Some things to check:
Did you save for web output? If you did, your jpgs should be RGB; not CMYK (which isn't supported by most browsers).
Did you optimize the filesize for web? Full size web optimized images should be no more than 100KB (less is better).
Did you insert your images into a division layer behind other page elements? If so, it could be there but not visible.
For better answers in these forums, post a URL to your test page so we can see it in a web browser.
I did save the image for web and that seem to do the trick.
Thank you for your input.