I asked Ring Lardner the other day how he writes his short stories, and he said he wrote a few widely separated words or phrases on a piece of paper and then went back and filled in the spaces.
Total freedom can be absolutely limiting. Everyone who has ever stared as a blank canvass or a blank piece of paper can relate to that. You could draw/write everything, so it becomes impossible to get any idea.
In these situations it can be very helpful to restrict you freedom. There are many ways to do so. Classically, you would take a dictionary, flip through the pages and pick any word — what ever you want to paint or write has to do with it (you can repeat the process to make it easier/harder).
But working on your own makes it hard to compare your works with others. However, given the huge amount of artists on the web, there are many groups who offer “assignments” or “missions” to restrict (or free) your creativity. For example, if you want to have photograph assignments, “Mission 24” and “Class With Dave” (both on flickr) offer assignments you can follow. You can do them alone or contribute to the current ones and compare your works with others. Note that these assignments would work for painters and writers as well (although you could not share and compare your works).