Currently, not much to do. A bit of hacking gets results that aren't really satisfying.
\documentclass[colspace=25mm, subcolspace=0mm]{tikzposter}
\usepackage{lmodern}
\usepackage{blindtext}
\usebackgroundstyle{Rays}
\begin{document}
\begin{columns}
\column{.32}
\block{Walzing Wombat}{%
Wombats love the walz, ducks dance dixie
}
\column{.32}
\block[linewidth=0pt,bodyinnersep=0pt,roundedcorners=false,]{}{%
\centering
\includegraphics[width=.95\linewidth]{example-image}
}
\column{.32}
\block{lurking lizard}{%
sassy salamander
}
\end{columns}
\end{document}