Do you think a writing can always benefit from the use of a visual illustration? If so, how to find for free or buy an appropriate picture, photo, drawing etc.? What are practical recommendations?
It depends on how you want to use it. If you're just looking for inspiration, Deviant Art is a good place to start, or someplace else if your interests have their own art site. On the other hand, if you're looking for art to put in your story that's going to be different, though believe it or not Deviant Art is a great place for that as well.
Now, if you're just look to put the picture on your website and link back to the artists, they maybe willing to let you use an existing piece for free. On the other hand if your looking to put it in a book things are going to be more complicated.
Quick switch of hats, I used to be an editor and publisher of a small press magazine, Deviant Art was the first placed we looked for artists. We almost never used existing art we found on the site (though there was one cover) but we were always willing to commission something from an artist.
Most quality artists are willing to talk about commissions, but the prices might be more then you expect. Most of our covers cost around $150 each, some more then that, and to be honest we got some fantastic deals.
So, here are some things to keep in mind when you're talking with an artist:
Also, depending where you get published, you might not have say on the illustration for your story. That is usually the editors job...
Yes, illustrations can have a good effect on your writings, just don't turn your book into glance journal.
Basically, your publisher could take illustrating work on himself, but if you want to furnish your book yourself, here are some advices:
1. Find a good image for cover of your book.
This is the most important illustration of your book, perhaps, because it brings readers attention. Pictures suitability really depends on genre. I prefer unusual beauty drawings with a mystery slice like works of Salvador Dali. You can find great book covers here
2. Inspire some picture artists
Present your book to some artists around you, and maybe they'll become inspired so much that will draw or make a photos specially for your work. With our digital era it is not that hard.
3. Look on image art resources (but not image hostings)
This advice should be combined with second. Yes, Flickr and deviantART are here for you. You've found a great picture? Well, just ask an author, and promise to reference him. Most of them will gladly give you rights to use their works free or for a very low fee.
4. Don't use prohibited images even if you liked one very much.
Courts are bad for business.