Great "short" answers!
Just to expand into the general concept that will answer this question in a more understandable way:
Wherever you have a macro that expands into a length for instance: \parskip, \linewidth, ... Or in this case \textwidth you can create another length just by setting a floating point factor before the macro.
For example:
0.78\parskip
2\linewidth
- ...
or in your case
0.45\textwidth
are valid values for the parameter width in includegraphics
You can take a look at the first lines of the wiki article:
http://en.wikibooks.org/wiki/LaTeX/Lengths