IMO, generating UML diagrams for code that you have written yourself is kind of pointless. The value of UML is as a design aid and to a lesser extent as a reverse engineering aid. If you are intent on generating code documentation for existing well-understood code, Javadoc is probably better value than UML, and a lot less work.
The other problem is that generated UML class diagrams tend to be ugly because they tend to include more detail than is necessary, and because you need a human eye to lay things out decently. The same probably goes for sequence diagrams.
But if that hasn't convinced you, these links should help you do it.
(Disclaimer: scraped from this closed Question - UML automated generation for Netbeans 7.1)
Finally I need to take the generated GUI as an image.
Take a screen shot of the GUI using the tools provided by your OS.