I'm fairly new to rails, but I think I can help you.
I think you wan to use RMagick's from_blob flunction. It takes an
image as binary data and gives you an Magick Image object. Like so:
There are a number of ways to resize an image, but I have been using
change_geometry to maintain aspect ratio.
I don't remember the exact syntax, but the docs should be enough to
get you on your way.
(I'm not sure if this works, but it should be close)
image = Magick::Image.from_blob(@params['picture_file'].read).first
image.change_geometry("400x300"){|cols, rows, i| i.resize!
(cols,rows) }