In order to avoid copyright infringement, here are the simple rules you need to remember about. It's an excerpt from Terms and Conditions on my website.
The paintings students produce as a result of taking Krzysztof Kowalski’s video tutorials or based on Krzysztof's free reference photos are considered legally to be ‘Derivative Works’ and as such the copyright remains with Krzysztof Kowalski and any other photographer whose work has been used for these.
Students may share photographs/scans of the paintings they create based on Krzysztof’s video tutorials and free reference photos online but they must always credit Krzysztof Kowalski whenever the work is displayed (including online).
Students may sell the physical paintings they create based on Krzysztof’s video tutorials and free reference photos (for instance in a local art exhibition) but they must always credit Krzysztof Kowalski whenever the work is displayed for sale.
Students can reproduce their own paintings provided the reproduction is for their own non-commercial use (for instance they can print it onto a Christmas card sent to friends and family) however, they must credit Krzysztof Kowalski along with the reproduction.
Students may not reproduce their own versions of these paintings for commercial use (for instance as an illustration or onto greetings cards they intend to sell).
Students should use the following wording to credit Krzysztof Kowalski:
For a painting completed via a video tutorial: ‘Painting based on a tutorial by Krzysztof Kowalski – www.esperoart.com’.
For a painting completed via a free reference photos: ‘Painting produced using a photograph provided by Krzysztof Kowalski – www.esperoart.com’.