I happily use LaTeX package letteracdp to write all my letters. There’s just one problem: it lacks a way to put a letterhead different from the one encoded in the source file.

A simple solution is to use eso-pic to draw the letterhead as a watermark in every page. The code to do this is the following.

\makeatletter
\AddToShipoutPicture{%
  \setlength{\@tempdimb}{\oddsidemargin}%
  \addtolength{\@tempdimb}{1in}%
  \setlength{\@tempdimc}{1\paperheight}%
  \addtolength{\@tempdimc}{-1in}%
  \setlength{\unitlength}{1pt}%
  \put(\strip@pt\@tempdimb,\strip@pt\@tempdimc){\parbox{\textwidth}{\centering%
    \raisebox{-18pt}[0pt][20pt]{\includegraphics[scale=.09]{sissa.pdf}}\\%
    \textbf{SCUOLA INTERNAZIONALE SUPERIORE DI STUDI AVANZATI}\\%
    Via Beirut 2--4, 34151 Trieste (Italy) tel: (+39)04037871 - fax: (+39)0403787249%
    }}%
}
\makeatother</pre></blockquote</p>

Of course this snippet can be used in any situation, not only with letteracdp. You only have to download Sissa’s logo (converted from the SVG obtained here).

For future reference, here are two example of simple letters in Italian and in English.

• Example of usage of letteracdp in Italian (sources, PDF).
• Example of usage of letteracdp in English (sources, PDF).