Have a look in /home/QtPalmtop/pics/ on your Zaurus, you can use those png's as templates for size.
To make changes to programs, you have to have
* The source code for the program
* A cross-compilation toolchain on your PC OR a compiler on your Zaurus
* Good knowledge of the programming language the program is written in (most likely C, C++ or Java)
...all of which is of course beyond the scope of this thread, search the forum for more info. If you're not familiar with programming it's going to be a lot of work, don't let that put you off though.