First let me say that except for my text editor, there isn't a tool I use more often that Screenshot Captor. I even uninstalled a commercial program that the company bought for me because this was so much better.
My suggestion. When printing a captured screen shot I often want it to be larger or smaller than the default size, but when you go to print->specified size and change the width you have to guess at the height so that you maintain the aspect ratio. If would be nice if you added another checkbox that (when checked) made the height change automatically when the width changes and vice versa based on the ratio between the two numbers. You already do it automatically when you scale the graphic to print using "fit to page", so it shouldn't be too hard to do it dynamically when a user specifies the size.