I use Adobe Fireworks. I started using it way back in the early 2000's when it was still Macromedia Fireworks. I tried several graphics creation applications and i found it to be the easiest for beginners...for me anyway.

Having said that...its not free or cheap. GIMP is probably the most robust freeware graphics app available. If you are starting from scratch you might as well start with good freeware. http://www.gimp.org/

Also...there is no need to reinvent the wheel these days. When i got started with user interfaces it was impossible to find transparent png files on the web. There were GUI templates out there but the touchpanel hardware limitations and my limited skills made it painful. Things are much easier today.

Go here [Link: guijaboard.com] On the right hand side under "GUI Professionals" you will find links to a bunch of GUI sites. You can get full templates or just graphics to build your own and/or icon sets that are "AV / Integration" specific.