Hi,
First I'm wondering why you type
miniMap.setViewport(sf::FloatRect(0, 0, 1, 1));
since the default behavior of a View is taking all the window. Plus, calling
mWindow.setView(miniMap);
in the main loop is not essential, you can do it once.
My first reflex would have been to call
sf::View::reset()
or directly use the constructor
sf::View miniMap(0, 0, imageWidth, imageHeight);
Hope this helps you.