first of all awesome plugin. I have been looking for something like that for a couple weeks now until today when I finally stumbled upon your plugin. It works just fine.
But I guess you can still improve it. Two things I would suggest are:
1. Change the fonts you use to normal WordPress fonts. (Just a suggestion)
2. This is actually a feature request. When you have you post thumbnail size set to crop, your plugin actually allows me to resize the frame, that actually defines the crop area, proportionally, which is great. But it would be even better, when this little fram would turn green once the amount of pixels inside the frame is higher than the amount of pixels you need for a non-stretched image in that postthumbnail size.
Example: Let's say my postthumbnail size is called "small-tile" and has a size of 100x200 pixels with cropped enabled.
Now I can change the crop area, but I don't actually see when I have at least 100x200, so I could end up with a 50x100 area selected which means the image is getting scaled up and so loses quality.
If you could just add this little feature the frame would turn green once you reach at least 100x200 and then stays green. So if you resize the cropping-frame to 50x100 it would stay white and if you resize it to 100x200 or above it would turn green and stay green, because this indicates that the image area that we are about to crop is actually big enough for the system so that it doesn't have to stretch the image.
Maybe you can also add a field that shows the actual size of the selected area. If it is too hard to turn the cropping frame green you can also just color these fields green once you reach at least the size of the actual thumbnailsize you are working on.
Hope that makes sense. Because I showed your plugin to a client today and you know how clients are. He cropped a smaller portion of the image, that portion was smaller than the actual postthumbnailsize and so the cropped image got scaled to fill the size, which made it blurred and ugly.
I will wait for you answer and then I will actually donate because I would like to support you and your great plugin.
And don't get me wrong these are just feature request or ideas for improvements.
Looking forward to your answer.