Some neat tips on Creating image thumbnails with ImageMagick.

I do this inside Ruby on Rails with RMagick. RMagick is sometimes a bit "interesting" to install, but do perservere, because you can then do some really neat things with it, like:

          ox = image.columns
          oy = image.rows
          if (ox < oy)
            nx = 200
            ny = (oy * (200.0 / ox)).to_i
          else
            nx = (ox * (150.0 / oy)).to_i
            ny = 150
          end
          image.sample!(nx,ny)
          image.crop_resized!(200,150)

This little code snippet will resize your images to be 200x150, and will recenter them if their aspect ratio is too long.