This has been working before, see e.g. here for reference. The png picture is in the same folder as the latex document. Do I miss anything?
\documentclass[12pt]{article}
\usepackage{fancyhdr}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{epstopdf}
\usepackage{mathtools}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{url}
\usepackage[parfill]{parskip}
\usepackage{enumerate}
\usepackage{framed}
\usepackage{xcolor}
\begin{document}
Start with nice text...
\begin{figure}[h!]
\centering
\includegraphics[width=0.5\textwidth]{Temperatur.png}
%\caption{}
%\caption{Schema des Verkehrsflusses \protect\\\hspace{\textwidth} aus http://www.mathe.tu-freiberg.de/~bernstei/HMI/Verkehr.html.
%\label{}
\end{figure}
\end{document}
The first part of the error:
