Fix MacOSX

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Fix launcher section in the README.html

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Only copy ImageJ.png for Linux

... because it is needed for the desktop icon on that platform.

While at it, change the location to images/icon.png so that the
launcher can use it (Wayne communicated that he actually liked the
images/icon.png location I suggested earlier better after this
developer changed the launcher to accept the icon at the project root
directory).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>