This page mainly lists major Touhou art dump sites. However, keep in mind that many other Touhou sites have their own image dump sections, as well.
Pixiv is a Japanese online community for artists, but also includes an English language interface. As of May 2010, the site consists of over 2 million members, over 11 million submissions, and receives over 20,000 submissions daily. As of January 2010, the website is visited approximately 34 million times daily, and about 1.5 billion times monthly as of May 2010. Pixiv aims to provide a place for artists to exhibit their illustrations and get feedback via a rating system and user comments. Works are organized in an extensive tag structure which forms the backbone of the website.
Most Touhou fan-art that finds its way to the English speaking reaches of the internet is published via Pixiv.
Danbooru (Japanese for cardboard box) is an imageboard-like website focused on anime-styled content. It's different from imageboards such as 4chan or 2ch, as posts there have no expiration date and categorized by tags. It's more like a gallery in that sense.
About one fourth of all images are Touhou fan content, and as of May 2011 there are about 830,000 images.
Most images at the site are just reposted from Japanese sites such as Pixiv. However, since they lacked an English interface until recently, Danbooru is more convenient for those who don't know Japanese.
A considerable amount of the content is sexually explicit, making this site not suitable for minors. Even when restricting search queries by rating, one should be aware that some ad banners could be explicit, too.
Gelbooru is an imageboard-like website focused on anime-styled content, similar to Danbooru. Originally created as an alternative after the initial success of Danbooru, it features much the same content and requires no registration for access to its full services. However, being less restrictive, Genbooru also has little to no image quality standards and is extremely NSFW in ads and content.
Safebooru is an imageboard-like website also focused on anime-styled content, like Danbooru. It was originally created as a replacement for Danbooru's defunct "safe search" mirror and uses the same software as Gelbooru. As its name implies, it is ideal for users wishing to search for SFW Touhou.
DeviantArt is a popular site for artists to upload their works, ranging from drawn art, to fanfics, photography and cosplay, and also features social networking services, such as journals. As of August 2010 the site consists of over 14.5 million members, and over 100 million submissions, and receives around 140,000 submissions per day. As perhaps the largest artist resource of its size in the Western world, Touhou is a popular subject for those artists and users without access to or understanding of Pixiv.
4chan is a imageboard created to serve as an American based counterpart to Futaba Channel. It has a large amount of fans of Japanese culture and Touhou, particularly as part of its /jp/ board. However, it is extremely NSFW, so caution should be taken when browsing.
Pooshlmer is one of the leading Touhou imageboards and communities among the English fanbase.
Cirnos is a relatively new imageboard, also featuring sections for art sharing and IRL events.
Desuchan's Touhou imageboard. Usually NSFW.
Coolier is a Japanese art site mainly featuring oekaki style drawings.
KonaChan is a wallpaper/background imageboard.
Zerochan is an anime imageboard-like website, sorting its content using a tag system. It has a sizable presence of Touhou fanart, though many images are reposted from Pixiv.