Starting work on an
illustration, we have a specific goal.
Even if it's just inspiration, you are developing your own skills and these skills are specific, since the reference determines the choice of material and styling possibilities. For example, reference for watercolor is far from the same as reference for graphic
sketches. For watercolor, you might choose something more multicolored, harmonious in color combinations, with interesting lighting. In a word, what can be well beaten with paints in combination with water. For graphics, it makes sense to choose even a black and white reference, which simplifies the transfer of volume. But speaking now about watercolors and graphics, I mean photos.
It is possible that you were inspired by the work of another artist. What in this case? Will it be your job? Yes. Can it be used for commercial purposes? No.
Since you are using someone else's intellectual work. The technique and/or the theme of the illustration that you copied was created by the artist through his own creative searches, it took not only the time of the work itself, but also the whole path that led to it. Of course, putting yourself in the place of this author, you will understand how unpleasant this is. Perhaps you yourself found yourself in a situation where your work was “stolen”.
Thus, copying someone else's work of art is your experience, training, test of technique, but not your property.
If you are going to post this copy on social networks, be sure to tag the author and indicate in the text that the work is a copy and does not aim to sell it.