On my website, you can find quite a bit of artwork:
1. All the internal illustrations from all the T3I books published in the United States are on my website. Just go to the STORIES section and click on the book title in which you're interested. Besides the US illustrations, I've also posted the UK illustrations by Roger Hall for those books that contained those. These were graciously provided by Ian Regan.
2. The covers for the first 15 books from the US were wraparound covers and you can find them under the section entitled THE COVERS. In addition, wraparound art for the covers graced the UK covers up through #28 ("Deadly Double"), and you can find them at the same location.
3. Lark Pien has shared several of her sketches that she refers to as "doodles." They 're quite good, and I'm sure you'll enjoy them. You can look at them here.
4. Martha Schwartz, a fellow fan of the series, has sent me several pieces of artwork over the years. You can find more about her here.
In addition, my friend Seth Smolinske has posted T3I fan artwork on his site as well. Just go to www.threeinvestigatorsbooks.homestead.com and look for the section entitled "Fan Art and Writing."