to answer myself:
it turned out that \myheader is left undefined when no option is passed. so i
just define it (empty) and the following works
--------------------------beamerthememytheme.sty---------------
\def\myheader{}
\DeclareOptionBeamer{header}{\def\myheader{#1}}
\ProcessOptionsBeamer
\mode<presentation>
\setbeamertemplate{background canvas}{
\includegraphics[width=\paperwidth]{header\myheader}
}
\mode
-------------------------------------------------------------------------
porting lyx to tex would be a great summer of code project btw...
