By your description, your works could be considered "derivative," that is, based on or derived from other original works. In Australia and in the United States the copyright owner (in this case, the photographer) has a right to control the making of derivative works.

Here is how this can play out. Several years ago, artist Jeff Koons based one of his sculptures on a photograph he had purchased in a gift shop. The photographer later sued Koons and was awarded several hundred thousand dollars. Painter Robert Rauschenberg also had to pay damages when a photographer sued over his use of a photograph in a Rauschenberg collage. So, if your use is discovered and a court determines it is an infringement, you will have a problem.

It's impossible to say how much copying or similarity is permitted, since these determinations are done on a case-by-case basis. But if, as you say, there is a strong similarity between your work and the original photograph, you could land in hot water. In the U.S., a legal principle known as "fair use" may provide a basis for your defense, but it is not reliable or predictable and can be used only after you're in a lawsuit.

Of course, there is an additional factor at play -- whether the infringement will be discovered by the copyright owner. The examples mentioned were all lawsuits against big-name artists, and that's probably no coincidence -- but then, you didn't mention how far you've come on the road to fame. If your work slips under the copyright owner's radar screen, then you're unlikely to have a problem.