A: 

html does not support dragging/dropping. RIA's and some AJAX/javascript pages may allow dragging of images ... e.g a shopping cart.

Rick J
+6  A: 

You can drag the image (like your favorite unicorn picture, I assume) to an explorer window and it will be saved there.

sth
Also, you can drag an image from an explorer to the browser, and it will be displayed there. So actually it *does* support drop.
troelskn
So, it can be dragged only on "explorer window"?!
Dhana
No. When he said Explorer Window, he actually meant any place on your computer showing files (My Computer, Desktop), I'm sure there's more, I just cant think of them right now.
Macha
When I try to drop on mspaint or word.doc, it doesn't work.
Dhana
Open "My Documents" and try to drop it there...
sth
@LittleBoy mspaint and word failing to accept an image dragged from firefox is the fault of the receiving programs. there are programs that understand it and accept the images appropriately — explorer is one such program — so firefox does allow drag and drop, it's the other programs that don't understand firefox's drop.
Blair Mitchelmore
+5  A: 

It does allow you to drop. Drop to explorer/desktop and it pastes the picture, drop to a input field or the address bar and it drops the image URL.

Macha
A: 

You can disable this in the firefox configuration.

http://www.google.com/search?q=firefox+image+drag

thijs