I added a long image to an HTML message, but it is not shown in MS Outlook even if I allow images

Yes, there is a physical limitation that is related to Outlook 2007 and Outlook 2010's use of the MS Word rendering engine. Images longer than 18 inches (1728 pixels) are not properly displayed (they are either truncated or not shown at all).

Really, you should never run into this limitation because an image that long simply does not make sense :-)

The only case in which you would have an image that long is the scenario of a message that was created in a graphic program. That is: the entire message is an image. That's a practice that should always be avoided as that kind of message is typically flagged as SPAM by the email client, and placed automatically in the Junk/Spam folder.