Pictures in boxes with rounded corners

watermark graphics is a tcolorbox option which uses graphics file as box background. If no text is desired inside upper or lower parts, height option fixes box size.

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\begin{document}
\begin{tcolorbox}[enhanced, watermark graphics=frog, watermark overzoom=1.0, title=example]
\lipsum[1]
\end{tcolorbox}

\begin{tcolorbox}[enhanced, watermark graphics=frog, watermark overzoom=1.0, notitle, height=5cm]
\end{tcolorbox}
\end{document}

Another option could be to use clip upper option, which adjust upper part to interior size and form. In this case \includegraphics command forms the upper part. Following code is copied from page 166 in tcolorbox (v3.90) documentation.

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\newcommand{\mygraphics}[2][]{%
\tcbox[enhanced,boxsep=0pt,top=0pt,bottom=0pt,left=0pt,
right=0pt,boxrule=0.4pt,drop fuzzy shadow,clip upper,
colback=black!75!white,toptitle=2pt,bottomtitle=2pt,nobeforeafter,
center title,fonttitle=\small\sffamily,title=\detokenize{#2}]
{\includegraphics[width=\the\dimexpr(\linewidth-4mm)/2\relax]{#2}}}

\begin{document}

\mygraphics{frog}
\mygraphics{lion}

\end{document}

update tcolorbox more recent versions include command tcbincludegraphics which combines an includegraphics command inside a tcolorbox. Therefore previous examples can be simplified. More information can be found in section 11 Inclusion of Boxed Image Files. Here you have some examples:

\documentclass{article}
\usepackage[most]{tcolorbox}
\usepackage{lipsum}

\tcbset{colframe=red!70!black, 
    size=tight, boxrule=1mm, arc=2mm, auto outer arc,
    nobeforeafter}
\begin{document}
\tcbincludegraphics{frog}

\tcbincludegraphics[width=.5\linewidth, graphics options={width=3cm}]{frog}
%
\tcbincludegraphics[width=.5\linewidth, hbox, graphics options={width=3cm}]{frog}
\end{document}

Since tcolorbox is implemented using tikz, this is actually simpler.

\documentclass{standalone}
\usepackage{tikz}

\begin{document}
\begin{tikzpicture}
\sbox0{\includegraphics{example-image}}%
\path[clip,draw,rounded corners=1.5cm] (0,0) rectangle (\wd0,\ht0);
\path (0.5\wd0,0.5\ht0) node[inner sep=0pt]{\usebox0};
\end{tikzpicture}
\end{document}