Take a look inside of
/usr/share/applications/gimp.desktop file. You will see that the value of
Image field is set to
gimp. So, there is not given an absolute path for the GIMP icon. This means that the system will look for the icon most probably in
/usr/share/icons. But there are more icons for GIMP, and the right icon will be chosen depending on the theme and on the resolution at which this should be displayed:
Finally, in the Launcher is chosen (most probably - depending on your settings)
/usr/share/icons/hicolor/48x48/apps/gimp.png file and in the Dash is chosen
/usr/share/icons/hicolor/256x256/apps/gimp.png file. If you will open these files, you will see that these look exactly like in your images.