Don't mix
\\
and
\
in the String Path.
Maybe it should look like:
string strImagePath = pptdirectoryPath + currentSlide + "_" + shape.Id + ".png";
The pptdirectoryPath string has also a backslash to much.
A single backslash is a so-called 'escape' character. This is used to include special characters like tab (\t) or a new line (\n). In order to specify a backslash in the case of a path you will have to 'espace' the single slash using the escape character, which means that you will have to write a double backslash.