How can GIMP 2.8 have two different icons simultaneously?

In Unity dash GIMP 2.8.10 has a icon and uses a different one in launcher.

GIMP in Dash

GIMP in Launcher

I searched for GIMP in "File System" and got this.

How can this be done while packaging other apps?

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:

locate gimp.png
/usr/share/app-install/icons/gimp.png
/usr/share/icons/HighContrast/16x16/apps/gimp.png
/usr/share/icons/HighContrast/22x22/apps/gimp.png
/usr/share/icons/HighContrast/24x24/apps/gimp.png
/usr/share/icons/HighContrast/256x256/apps/gimp.png
/usr/share/icons/HighContrast/32x32/apps/gimp.png
/usr/share/icons/HighContrast/48x48/apps/gimp.png
/usr/share/icons/hicolor/16x16/apps/gimp.png
/usr/share/icons/hicolor/22x22/apps/gimp.png
/usr/share/icons/hicolor/24x24/apps/gimp.png
/usr/share/icons/hicolor/256x256/apps/gimp.png
/usr/share/icons/hicolor/32x32/apps/gimp.png
/usr/share/icons/hicolor/48x48/apps/gimp.png

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.