Wolfgang,
At the present time there is no ability to simulate textures in previews.

I would suggest doing that in Photoshop or another PP program that enables you  to add textures to images.  Just copy the file and make a textured version for previewing by you customers, but use the original file to print from in Qimage.

An even better suggestion is to have some examples available of the same photos printed 5 ways as samples. #1 Matte paper,
#2 Glossy or semi gloss, #3 Gloss canvas #4 Matte canvas,#5 Semi gloss canvas, and let the customer decide which 'texture' they like.
Seeing in real life is even easier than trying to imagine the texture on the screen  The samples will speak for themselves and should make choosing much easier 
 
If a customer cannot visualize what his or her own photo will look like, well .....
David

That would be an interesting concept but since the majority are using the program for our  printing needs and not for displaying previews to customers, I don't think it would be a necessity for most users, and would only start to bloat the program with extra baggage so to speak.Certain paper textures would be hard to duplicate on screen, and there are so many  out there that it would be difficult to represent them all.Canvas is relatively easy to see.
It is really easier to do it in a program that already has textures that can be added like Corel PSPP X3 or AdobeCS5.
You might just e-mail Mike and see what his reaction would be.
He is usually very receptive to user requests for features and if he feels it would be beneficial, he might just take you up on your request