How to find specific files, installed by applications
Found them on Ubuntu 16.04 in /usr/share/dia/shapes. Ubuntu or Lubuntu shouldn't make a difference. You can find out where files are installed with the help of
dpkg-query -S .shape
to look for specific patterns. This shows that all .shape files are in /usr/share/dia/shapes. They are grouped however in many subdirectories.
Note however that for each and every .shape file, there is a corresponding .png file, most likely to represent the shape in the menu:

See also man dpkg-query
-S, --search filename-search-pattern...
Search for packages that own files corresponding to the given
pattern. Standard shell wildcard characters can be used in the
pattern, where asterisk (*) and question mark (?) will match a
slash, and blackslash (\) will be used as an escape character.
If the first character in the filename-search-pattern is none of
‘*[?/’ then it will be considered a substring match and will be
implicitly surrounded by ‘*’ (as in *filename-search-pattern*).
If the subsequent string contains any of ‘*[?\’, then it will
handled like a glob pattern, otherwise any trailing ‘/’ or ‘/.’
will be removed and a literal path lookup will be performed.
This command will not list extra files created by maintainer
scripts, nor will it list alternatives.
To find out where an application installed its files, use (in this case):
dpkg-query -L dia-shapes
Finding out with synaptic
Also Synaptic offers help how to find out.
- Search for `dia, right-click on the package
- Click on "Installed Files" (tab)