How to write multiple lines as watermark?... and images with transparency?

Using the draftwatermark package, you can use a \parbox, or a minipage, to have the text spanning several lines:

\documentclass{article}
\usepackage[a6paper]{geometry}
\usepackage{draftwatermark}
\usepackage{url}
\usepackage{lipsum}

\SetWatermarkText{\parbox{12cm}{%
  Mika Ike \\
  \url{[email protected]} \\ 
  At My house}}
\SetWatermarkScale{.5}
\SetWatermarkColor{red}

\begin{document}

\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]

\end{document}

In the case of opacity for images, the draftwatermark package apparently doesn't provide many possibilities.

As an alternative, I'd suggest you to use the background package; since this package internally uses TikZ, you can easily control the attributes (opacity, color, position, etc.) for the background material. A little example:

\documentclass{article}
\usepackage[a6paper]{geometry}
\usepackage[scale=2.5,opacity=1,color=red]{background}
\usepackage{url}
\usepackage{varwidth}
\usepackage{lipsum}

\backgroundsetup{
contents={\begin{varwidth}{8cm}{%
  Mika Ike \\
  \url{[email protected]} \\ 
  At My house}\end{varwidth}}
}

\begin{document}

\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]

\end{document}

The varwidth environment was used to have a box of adjustable width allowing multiple lines of text; a minipage or a \parbox could also have been used instead.

And, using an image:

\documentclass{article}
\usepackage[a6paper]{geometry}
\usepackage[scale=1,opacity=0.75,angle=0]{background}
\usepackage{url}
\usepackage{varwidth}
\usepackage{lipsum}

\backgroundsetup{
contents={\includegraphics[height=\textheight,width=\textwidth,keepaspectratio]{ctanlion}}
}

\begin{document}

\lipsum[4]\lipsum[4]
\lipsum[4]\lipsum[4]

\end{document}

CTAN lion drawing by Duane Bibby.

One solution is to use \shortstack{} which can be found in the manual of the draftwatermark package.

Example:

\documentclass{article}
\usepackage{lipsum}
\usepackage{draftwatermark}
\SetWatermarkText{
  \shortstack{
        \includegraphics[scale=0.8]{ctan_lion_600.png} 
  }
}
\SetWatermarkScale{0.15}

\begin{document}
\lipsum[1]
\lipsum[2]
\lipsum[3]
\lipsum[4]
\end{document}

Example:

\documentclass{article}
\usepackage{lipsum}
\usepackage{draftwatermark}
\SetWatermarkText{
  \shortstack{
        this is the first line  \\[1em]  second line 
  }
}
\SetWatermarkScale{2}
\SetWatermarkColor[gray]{0.5}

\begin{document}
\lipsum[1]
\lipsum[2]
\lipsum[3]
\lipsum[4]
\end{document}