Merb and rmagick thumbnails

I’m playing around with a merb app to upload images. The upload works
fine and the images are copied from the temporary directory into an
uploads directory in the merb root with the original filename. now i
want to create thumbnails using RMagick, but when I add the code, I get
an internal server error when i try to load page:

Internal server error 500
uninitialized constant Photos::Photo

You can view my model, controller and views at here:
Thank you for your help