I’m assuming that you’re asking from the context of legal usage of online imagery/art…
So long as you’re using a concept image you find online for educational purposes only – i.e. not for commercial gain – you can use whichever image from the web that you like. If you post something you create from that concept to the internet, I recommend attributing credit (up to and including a link) to the concept artist; it’s the smallest courtesy everyone should do. To go a step further, you can always reach out to that artist to ask permission. In my experience artists are more than OK to let you use their art for creating something else from it.
*Again, all of this is based on you not trying to gain financially from the artists work.
That said, check out artstation and pinterest for seemingly infinite supply of monster concept art.
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