A quick look through some of the home brewed RPGs shows a mix of great artwork, but there is also some terrible imagery.
I know that a lot of independent designers are one-person operations, and there are only a very few people can be a great at all aspects of publishing (writing, editing, layout, artwork, marketing), but this doesn't mean artwork has to suffer.
Even if a game designer doesn't have artistic talent, or a doesn't have a budget to hire someone for the job, there is some awesome public domain art available.
I've done a quick search for "public domain fantasy images", and found enough links to get anyone started...as well as a few forum posts that link to new avenues for research.
The Google Search Result
(EDIT: A quick look at some of these images shows a few that I recognise from various sources and are probably not public domain...but you can narrow down the search with Google, perhpas limiting the results to those with "labelled for resuse", or "non-commercial" if you are planning to use it with a free game.
13 Art Sources
A couple of responses on Stack Exchange
The RPG Site
Even that terrible site RPG.net
...and that's just a cursory glance because I like to illustrate most of my own products.
A quick look at book layout ideas generates just as many results, but this topic requires a bit more work than just copying an image an sticking it on the cover of a book or adding among the words.