How do I handle using an image?

I need to use some images for my blog, but I don’t know what I can and can not use. I am right now using things that are in the public domain and under the creative commons license. I want to know what other images I can use…for example, can I take elements from a movie poster and combine them? What are some rules when it comes to copywright? Also, what are the rules with trademarks?