Image selection

Removes _selectedImage.
Rather than have `manipulateAndAssert` modify the `resume` global (from the function's perspective), how about having line 138 be something like `asyncCallback = manipulateAndAssert(executions.pop());`. Then line 151 in `manipulateAndAssert` is `return assertResultUndoAndAdditional;`.

There's no need to use globals/closures when functions that return things will do the job, right?