I don't know of an all-in-one solution, but PDF Converter will convert a great many things to PDF, including images from the Photos app (where screen shots reside).
You can then open the PDF in an app like PDF Expert or GoodReader (my two favorites) for annotation.
Or if it's just about quickly marking up a photo or screen shot with some shapes and text, you might want to check out Skitch. It integrates with Evernote (if you use that) and exports as png or jpg. Oh, and it's free. We use it a lot here on the forum.