Thanks for your interest in SkylineCMS.
As for your question. RMagick is the the gem that ruby uses to interact with ImageMagick.
It seems that there's a dependency missing. How did you install ImageMagick? And what version are you running?
We install ImageMagick with mac ports. It may help to look at this guide: http://rmagick.rubyforge.org/install-osx.html
Let me know how you get on.
Cheers, Luis