Watermark using geometry and extent

[tuxtux59]

Hi,
I found your gem pretty nice but I can't get what I'm looking for:
I would like to generate a 320x480 px image where the original content fit merged with your watermark.

I succeeded got a 320x480px with the following code:
has_attached_file :picture,
thumb: {thumb: '320x480>'},
convert_options: {thumb: '-gravity center -background white -extent 320x480'}

With this code I have a 320x480px image with original fit inside without ratio loss

When I had processor [:watermark] and replace '320x480>' by{ geometry: '320x480>', watermark_path: "path_to_watermark.png"} The compilator throws me an error due to unrecognized method gravity.

Have you some idea to get both image size and watermark?