Echo New Icon GUI Script Design Idea

In Echo Project we strongly recommend to create new icon using the echo-icon-new script which does some of the work, that an artist would need to do, behind the scenes. Today I've started thinking about writing a GTK version of the script so that the usage would be even easier. Here's my initial design idea of the GUI (click to see in original size):


Now, two issues arise:

• How good is this layout from usability POV?
• As I've never done GTK app, how to actually create this layout, preferably in ruby?

Both feedback for the first question and help for the second question is appreciated.