NGUI UISpriteAnimation - how to animate a set of sprites with different dimensions

I made 4 images of an animation. Each image has dimensions 512x512. because the contents of the images vary and because the Trim Alpha option is selected on the atlas, the resulting sprite dimensions are different from each other.

This yields undesirable results when animating a set of sprites. How do i keep the aspect ratio uniform?

I don't think the pixel snap option is what I'm looking for because the error starts at the import stage. Also, it looks like the sprite padding values are used just for texture packer information and now for rendering.