Under copyright law, as well as common courtesy, using someone else's work as a reference is considered making a derivative work, and is only allowed with permission, even if you are working in a different medium or making changes to it.
I do some figure drawing myself on occasion (poorly), so I understand the need to practice, and it isn't always possible to find your own model.
Because of this, I permit artists working in traditional media (drawing, painting, or sculpture; NOT photography, digital art, collage, mixed media, or video) to use my photographs as a reference for their own work, under the following limited conditions:
You may not sell the resulting piece or license it (free or otherwise) to be used for editorial or commercial purposes.
For example, you cannot sell the piece in an art show, nor can you give a blog permission to use it as a header graphic for an article, or allow it to be used to advertising a painting glass.
If you show the piece anywhere online or offline, it MUST be titled/captioned "After Richard Tallent, [year]", where [year] is the year I created the photograph (the first 4 digits of the reference number here on my web site). This convention has been used by artists for centuries.
Linking or tagging my social media account or web site is appreciated but not required. Please do NOT tag the model(s) unless you've asked them first.
You must anonymize the model's face or other identifying features (such as unique tattoos). My model release, which covers the model's release of their rights of publicity or privacy, does not extend to derivative works by other artists.
I don't allow my work to be used as reference for any art that promotes violence, hatred, bigotry, animal cruelty, homophobia, sexism, classism, or religious extremism, nor in any work that is sexually explicit or sexually charged.
The above may make it seem like I'm against artists copying my work, but I really DO love seeing what other people create! So, if you create something based on my work, please send me a photo of it via email. :)