Based on the given definition of \ULCornerWallPaper by wallpaper I would define a new command. This new command has an extra mandatory argument. Making the picture "clickable" is done by the command \href.
The package mwe provides the example picture and loads also blindtext for filling text.
\documentclass{article}
\usepackage{mwe}
\usepackage{wallpaper}
\usepackage{hyperref}
\newcommand{\ULCornerWallPaperWithLink}[3]{%
\AddToShipoutPicture{%
\AtPageLowerLeft{%
\parbox[b][\paperheight]{#1\paperwidth}{\href{#2}{%
\includegraphics[width=#1\paperwidth,height=#1\paperheight,%
keepaspectratio]{#3}}%
\vfill%
}
}
}
}
\ULCornerWallPaperWithLink{0.1}{http://tex.stackexchange.com/}{example-image-a}
\begin{document}
\blindtext[10]
\end{document}