When copying/tracing photos, drawings, painting etc that other people have done, you are infringing on their copyright. However, there is no reason why you cannot look at a photo and interpret that into your own original drawing. If you want to paint a tiger, then you are probably going to work from a photo.
When you learn how to draw you can capture whatever images you want to and will be much more satisfied with your completed silk painting. I know that it can take some time to learn to be proficient at drawing, but it is really worth the effort if you want quality silk paintings.