I built pb with: cd src / qmake / make / make install.
After the following changes in the install section of the pianobooster.pro the make install works fine.
When I compared the final binaries of cmake and qmake I determined file size (after strip command) differences.
Sorry have been away for some days.
I did nothing more. When I do a 'open with' in nautilus the file pianobooster.png located in the directory /usr/local/share/pixmaps is displayed as icon.