In general text you can force a LaTeX image to stay where it is placed in the code using the [!h] command in the figure environment.

A slight modification of this which allows the image to move to the top of the next page if there is an overhang is the [!ht] command.

For example:

\begin{figure}[!ht]
\centering
\includegraphics[width=3in]{imagefile}
\end{figure}


To add an image to a letter head you could use the fancyhdr package, such as in this example.