Unknown graphics extension: .1.png

The LaTeX graphics/graphicx package uses the first dot to find the extension. Package grffile changes the algorithm to check for known extensions (option multidot, enabled by default):

\usepackage{graphicx}
\usepackage{grffile}

Of course the obvious answer is just to rename the file so it doesn't have any dots in its name, but you can also try using {} braces for the name and get a good result. e.g.:

\includegraphics[width=0.5\textwidth]{images/{xxxx_0.1}.png}

Edit: moved the closing brace before the extension, as mentioned in the comments.

You can disable the filename parser by specifying the extension and file type as attributes. For pdftex that would be

\includegraphics[width=0.5\textwidth ext=.png type=png]{images/xxxx_0.1}