\documentclass{article}
\usepackage{graphicx}
\begin{document}
1 \includegraphics{a.b.png}
%2 \includegraphics{a.b}
3 \includegraphics{foo bar.png}
4 \includegraphics{foo bar}
{
\graphicspath{{nospace/}}
5 \includegraphics{a.png}
}
{
\graphicspath{{space here/}}
6 \includegraphics{a.png}
}
\end{document}