My university has a letterhead like this, and I created a package using the
textpos package. Here's very simplified version of a letter using the basic mechanisms I use in that package. The package itself is more complicated than this, since it allows for multiple graphical element options, and different coloured logos etc. But this should be enough to get you going:
% replace these with your \includegraphics commands to insert the graphical components
To whom it may concern:
Some comments on how to do this. In my letterhead, I place the text of the letterhead also as a graphical element rather than typeset it directly. This allows (i) for absolute placement of the text relative to the other graphical elements and (ii) the letterhead text to contain proprietary fonts that otherwise would need to be installed on users' machines. So in my actual letterhead, the elements corresponding the
\graphicThree are actual graphics, while
\graphicFour are in fact text. I also have PDF and EPS versions of all so that the package can be used with any TeX engine. But these are details that may not concern you.
One downside of this sort of letterhead is that you are faced with two options with respect to margins: make the left margin wide for the whole document or adjust the second page using
\newgeometry. This of course will force a new page, and it's not automatic.