While onaholes are perfectly legal unless the country has some religion-based or likeminded twisted law forbidding men to "waste sperm" or stick their penises into something else than women's vaginas (because of regular goat raping or whatever), onaholes can depict the outer appearance of women's genital organs, and therefor be censored by terms of contract (the few I have in mind: Apple, Facebook, Youtube,..) Likewise, domain registrars and web hosts may not allow sexual contents. That all is contractual and just means you get service from another, less butt-hurt company.
Now your question focuses on illustration, that's a tricky part. Onahole illustrations only go as far as Japanese law allows: censoring law requires to censor or pixellize.. "obscene" elements, like penises, pussy-slits and clitorises. Indeed, what's "obscene" or not is up to… anybody. The Japanese law basically trusts companies to find the balance between freedom of expression/commerce and obscenity. That's a legal "grey zone".
A great example: the winning illustration for the contest, and the actual illustration when the Onaho Fairy special edition was released:http://blog.onahole.eu/wp-content/uploads/2013/01/Fairy_Vote_001.jpghttp://i.imgur.com/fyFrRaO.jpg
Western countries, in general, don't require any censorship, as long as the content is not made available to underage people. Almost all countries (including Japan) introduced strong laws against pedopornographic content. That's good. But: depending on countries, things are ruled or judged as being pedoporn or not, the tricky point being fantasy, fictional and computer-generated content (where no child is actually being harmed; the point being if such content "creates pedophilia" or is neutral).
As of today, in the USA and Canada, it may raise suspicions, occasionally being censored or held for "obscene content". In Europe, there's less tolerance: I know in France there was a judgment years ago that ruled that you may not sell such content, even fictional. So doing business is ruled out. What about detaining such content ? I guess it's like in the USA: may raise suspicions about being a pedophile or whatever childkiller, since people in general aren't even aware onaholes exist (imagine the eyPost too long. Click here to view the full text.