You can use ordinary TikZ commands in a tcbposter
environment.
\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{poster}
\usepackage{lipsum}
\begin{document}
\begin{tcbposter}[
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=12cm,
},
]
\posterbox[
colframe = red,
width=13cm, height= 10cm
]{name=A,column=1}{\includegraphics[height=8cm]{example-image-b}}
\node[font=\sffamily\Large,rotate=90] at (12,9.6) {XYZ};
\node[font=\sffamily\Large] at (6,5) {XYZ};
\end{tcbposter}
\end{document}

Or you can position these nodes relative to the picture.
\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{poster}
\usetikzlibrary{positioning}
\usepackage{lipsum}
\begin{document}
\begin{tcbposter}[
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=12cm,
},
]
\posterbox[
colframe = red,
width=13cm, height= 10cm
]{name=A,column=1}{\includegraphics[height=8cm]{example-image-b}}
\node[font=\sffamily\Large,rotate=90,right=-5mm of TCBPOSTER@A] {XYZ};
\node[font=\sffamily\Large,below=-10mm of TCBPOSTER@A] {XYZ};
\end{tcbposter}
\end{document}

Or you can just use a table and \rotatebox
.
\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{poster}
\begin{document}
\begin{tcbposter}[
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=12cm,
},
]
\posterbox[
colframe = red,
width=13cm, height= 10cm
]{ }{\begin{tabular}{cl}
\includegraphics[height=4cm]{example-image-b} &\rotatebox{90}{XYZ} \\
XYZ &
\end{tabular}}
\end{tcbposter}
\end{document}

tcbposter
.