This may seem somewhat basic but I'm keen to learn how panel designers go about creating their textures.   Some panels are fairly crude - sort of simple 'paint' creations - and not very realistic looking.     Others seem to be almost photo-realistic in their rendering of the metallic textures employed.       How do they do it?

Yes, it's a basic question: We all know each panel-view has its own *.BMP and this file can be of anything. So there ya go! 

"WJI - I don't understand the point you're making"
I hear ya . . . sometimes I mumble 

Often when we d/l a photorealistic panel, we read the credits to whomever took the photographs of the aircraft's interior -- including the panel.

Whomever creates the 2D photorealistic FS9 panel converts these photographs to *BMP files (in the requisite format and size), creates all the blackholes for the designated gauges and uploads the finished product as a Photorealistic Panel -- meaning it uses real photgraphs of the panel and some views.

Therefore, I thought I gave a difinitive answer to your question:
"This may seem somewhat basic but I'm keen to learn how panel designers go about creating their textures.   Some panels are fairly crude - sort of simple 'paint' creations - and not very realistic looking.     Others seem to be almost photo-realistic in their rendering of the metallic textures employed.  How do they do it?"

I've done many myself and that's how I do it. Take any photograph or texture, rename it in the native format (panels are often just 24bit), and there ya go!

Try it I presumed you were referring to 2D panels

P.S. The same applies to outside aircraft textures; as noted, I thought it was pretty basic stuff. Have fun.