Beamer - setting image place in theme, specifying image in presentation

I'm trying to create my own theme for beamer. Right now it looks like this:

It looks very similar in sections, too. To put an image I use tikz. For now it looks like this:

\begin{frame}[plain]\gornagrafika{dna}\titlepage\end{frame}

While \gornagrafika requires:

\newcommand{\gornagrafika}[1]{\tikz [remember picture,overlay]    \node at ([yshift=2.5cm]current page.center)     {\includegraphics[width=\paperwidth,height=.45\paperheight,keepaspectratio]{#1}};}

Same in sections and subsections.

Is there any way to put similar code to inner.sty? To use something like

\titlepage{image}

or

\sectionpage{image}