require 'RMagick'
class Profile < ActiveRecord::Base
  belongs_to :user
  
  #image = the image passed from params[:image]
  #file_type = the confirmed filetype from the controller
  #current_user = various information about the current user
  def self.upload(image, file_type, current_user)
    #set default cols and rows for our two images that will be created from the uploaded user image
    img_size = {:main =>{:cols => 250,:rows => 375},
          :thumb =>{:cols =>80, :rows =>120}
         }
    #set base directory for the users image
    imgDir = "#{RAILS_ROOT}/public/images/user_images"
    #set the base name for our userimage
    userImg = " #{current_user.login}-#{current_user.id}"
    #loop through the image dir and delete any old user images
    Dir.foreach(imgDir){|file|
    if file.to_s.include?(userImg)
      File.unlink("#{imgDir}/#{file}")
    end
    }
    #read the image from the string
    imgs = Magick::Image.from_blob(image.read)
    #change the geometry of the image to suit our predefined size
    main_image = imgs.first.change_geometry!("#{img_size[:main][:cols]}x#{img_size[:main][:rows]}") { |cols, rows, img|
       #if the cols or rows are smaller then our predefined sizes we build a white background and center the image in it
     if cols < img_size[:main][:cols] || rows < img_size[:main][:rows]
      #resize our image 
      img.resize!(cols, rows)
      #build the white background
      bg = Magick::Image.new(img_size[:main][:cols],img_size[:main][:rows]){self.background_color = "white"}
      #center the image on our new white background
      bg.composite(img, Magick::CenterGravity, Magick::OverCompositeOp)

     else
      #in the unlikely event that the new geometry cols and rows match our predefined size we will not set a white bg
        img.resize!(cols, rows)
     end
      }
    main_image.write "#{imgDir}/#{userImg}.#{file_type}"

    thumb = imgs.first.change_geometry!("#{img_size[:thumb][:cols]}x#{img_size[:thumb][:rows]}") { |cols, rows, img|
       if cols < img_size[:thumb][:cols] || rows < img_size[:thumb][:rows]
      img.resize!(cols, rows)
      bg = Magick::Image.new(img_size[:thumb][:cols],img_size[:thumb][:rows]){self.background_color = "white"}
      bg.composite(img, Magick::CenterGravity, Magick::OverCompositeOp)

     else
        img.resize!(cols, rows)
     end
      }
    thumb.write "#{imgDir}/#{userImg}-thumb.#{file_type}"
    if FileTest.exist?("#{imgDir}/#{userImg}.#{file_type}")
    return "The file was written and its name is #{userImg}.#{file_type}"
    else
    return false
    end
  end
end

http://pastie.caboo.se/95581

Use this to upload and resize your images using RMagick. Code by nixnewbie.

Upload and resize with RMagick

resize, rails, ruby

Rails