Sorry, but I don't think this is possible at all.
The TWebBrowser component encapsules the normal Internet Explorer, which is in fact loaded as a seperate process and displayed in your application.
You do have
access to the
DOM in the internet explorer process which can pointing to
URI's of pictures, but you don't have
access to the rendering engine, and this is, where the pictures are loaded and displayed in fact.
So I fear you can't change the location where the picture is loaded from from an
URL to a pointer inside the application memory (where a stream or a TBitmap would be located at).
I think as a workaround you could write the picture from application memory to a file in the windows temporary directory and point the
url to that temporary file.