Quick answer is pretty much no, you can't publish or sell a photo without a model release. Most places (especially stock photography companies and competitions) will require model releases before you can sell your work. Now, I'm sure that not everyone gets model releases for say critique shots in the forums. However, the difference is that you're not making money off something like that, so you can easily take the photograph down if the person requests it.
There have been a number of discussions here on model releases (hope you're ready for some reading
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I realise that most of these links talk about photographing strangers, but that is because this topic comes up most often in the context of street photography. You will have a better idea of whether or not your friends and family would want their picture online. If you have doubts, then you probably have good reason not to post the picture online.
Hope that helps answer your question
