For your personal use, do whatever you please. Print images out and put on your wall, use for desktop wallpaper, put on a t-shirt etc.
For public use, such as in your blog or on your home page, please show the images as they are, but
- credit the artist, preferably with a link to this site;
- do not use them commercially, making money directly or indirectly from the publication;
- do not use them as design elements for a site, magazine, poster etc;
- do not alter the images other than resizing.
For commercial use, please get in touch.
Feel free to tattoo one of the artist's pictures. Send a photo if you wouldn't mind!
The tattoo artist may not keep the design and offer it to others as this would be commercial use.
Buying a work, even one that you commissioned, does not transfer the copyright or any commercial usage rights to you unless you explicitly purchase them.
If you want to use it commercially, such as on a book or album cover, you need to purchase those rights.