Today, I had to make up some quick wireframes for a possible freeware iOS app. I did a quick google to find a free wireframe tool, and boy, they're almost all pay to play, or not only are they pushing you to pay to play, the few free features are hidden behind a sign up you have to perform before downloading.
I did manage to find a decent one in wireframe.cc. Not heavy on the tooling -- I think it brags about being minimalist -- and there are a number of things you have to roll... yourself... every... time (like buttons, which are rects with rounded edges that you then drop a label on top of and move together later), but for free, it's very nice.
Here's an example of one screen from the insanely complicated (har har) UI I made.
(I'm helping a charity that distributes goods, and wants to ensure they haven't served anyone twice before everyone's been served, regardless of location. At the same time, we want to track an absolute minimum of information, and nothing that could be used as PII. So voila.)
I didn't see a way to use my own image, like a gear icon for settings, but for a mock-up, it's perfect. Quick and painless.