Reference image licenses.

If Shutterstock offers copyright free images - don't know, I've never used it - then you shouldn't need to worry about licensing; the terms and conditions of the website should tell you all you need to know. If you directly copy an artist's work, without their specific permission, you will be in breach of copyright; it is possible and permissible to make a stab at a great master and label it "by Thomas Greene, in the style of" e.g. "Botticelli", or, "after Gainsborough", or whomever.   If Shutterstock offers photographs - well, I don't know how good you are, but I could "copy" a photograph which looked VERY little like the photo.... I don't use other peoples' photographs, with the occasional exception of my brother's .... because, well: I'm not really able to.  You can always manipulate landscapes - drop something in here, leave something out there, move a mountain or two - to create your own version of a photograph.  But I wouldn't - I don't recommend, whatever the legal position, painting vistas you've never seen for yourself: I did it when I was very much younger; I was rather fond of elephants, and the only elephant I'd seen in the flesh was Rosie, of Bristol Zoo....  I wasn't selling back then (I was only 15, and basically copied pictures from the Brooke Bond tea albums)... Long digression there - to summarize: you'll be entirely safe if you never use anyone else's work as the basis for your own; you'll be pretty well fire-proof if you take images from a copyright-free site, and read their terms and conditions carefully; copyright won't normally matter for old master works, but you'd be foolish not to acknowledge their source - commercially and aesthetically rather than legally; and licensing doesn't normally come into this one way or the other - if it does, the website on which you've found the work will tell you that, and stipulate a fee. I am not a lawyer, be it said.  If in serious doubt, buy a copy of the Writers' and Artists' Yearbook, and read the advice there; or - find a friendly lawyer who'll give you advice in exchange, maybe, for a painting!  I think what I've said is basically accurate, though.   Obviously, forgery is a very different matter, but you'll not be wanting to go there!

Maybe contact the photo’s originator and ask his or her permission to sell a painting based on their photograph?  I try to do this even if I use a copyright free image from Pixabay. You need to ask whether the person who posts the photograph is the originator/ creator of the photograph because you could get in trouble if they have uploaded or copied  someone else’s work. 

Bendor Grosvenor has an update on Tate removing copyright on its existing web images here : https://www.arthistorynews.com/articles/7707_Images_fees_and_UK_copyright_law__a_breakthrough

Gov.UK has a good section that explains this topic very well. I use a site that I pay a small monthly fee, to use high quality images of my choice but they are mainly figures, a few animals not many landscapes. Still, it suits me for getting some practice doing portraits.  I also bought a second hand decent camera, with 40x zoom, which I now use to go out and take a lot of my own reference photos. I will also look for photos 100 years old as there are less likely to be any copyright issues but you still need to check. Yes the quality is poor but they are good for ideas and shapes and can be used for practice. Better still, set up your own still life and paint what's in front of you. We don't always have time to paint from life but should when the opportunity arises.