Searching way to print HTML web page to picture (BMP,GIF or PNG) file

Assume I have a (big) web page. How can I print this web page into a picture file
BMP, GIF or PNG format?

Note that a simple screenshot is not searched because the web page is NOT completely visible
and needs a lot of scrolling.

However the target picture should contain the full web page content.

Does someone know such a tool?

Alternatively a converter from pdf to BMP,GIF or PNG picture would be suitable as well.

Kevin