I use GUIGenie
http://www.guigenie.cjb.net/ for prototyping my interfaces. It is meant for JAVA, but it serves for me as image generation for my prototype GUIs. You can design your interface and later on you slice out each part of the GUI, each button (when highlighted, pressed, etc. also) and the whole form as well.
Here is a shot during runtime where I use my own debug panel especially meant for graphics (the two numbers and the string above in acknex font arent a part of the GUI, they are just for debugging, though):

With a bit scripting you can achieve everything: radiobuttons, buttons, dropdownlists, sliders, checkboxes, multiselection... everything you can see in the screenshot works well. Its automated and the whole panelstructure resizes with resolution changes.. the new filter flag for panels is SO NICE!

Maybe I should post GUIGenie in the starving developers list, hehe
sorry for spamming :-)