There are several points I want to make about plagarism as an artist & as a teacher.
As an artist, I have learn much from using the tool of photographs that I have taken & others have taken. If I use other people's photographs, I take one element- usually the position of the body, what a particular tree looks like, the angle of the building & use it as a reference to make a painting, illustration or puppet. I don't copy, but I do use photography as a tool.
Many professionals will tell that they use photography as a tool, just as some people use computers as a tool. As long as children learn other tools & skills in how to use the tools, I do not have an issue of students using photography (or anything else). However, they need to be taught how to use this tool appropriately & not just use it as an easy way out (just like computer graphics can be used to make a flashy image, when used as a crutch). Drawing from life is still crucial to develop depth perception, flexibility in thinking and visual spatial skills that working from a 2-d resource to a 2-d medium doesn't provide. Learning about shading, graduation & 3-D skill should be coincidently with using other tools.
Realistically, not all artists are fortunate to have available models all the time. Many artists don't have money or time to hire private human and animal models. Even artist who use models, still photograph their models to capture a movement or position that a model cannot hold for a long period of time or need the photo to replicate a position when the model comes back. This also gives them the flexibility to incorporate details when the model isn't available.
It is up to the teacher to show student how & why these tools are used. Throughout the ages, artists have duplicated various masterpieces to learn a style or technique. This can provide a young artist with exposure to help them decide what style, method & media to use in their own work. As long as they credit the origin of their material & do not claim that they are the creaters (ie. study of The Dancer, by Degas), I don't have issue with it. They should be informed what type of work is good for practice & what colleges & jobs look for when developing their materials.
Comic book publishers & animation studios, by the way, do require seeing in portfolios the ability to:
--copy previously existing characters exactly: penciled, inked & colored (attributing the resource)
-- original characters
--basic life drawing skills
--Concepts (original & ability to carry through)
--being able to carry through other people's creations consistantly
-- storytelling skills
--use of various media
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