I try to place the rendered image inside the UV Editor window so it fits its borders precisely. Unfortunatelly it isn't possible using a Mouse Wheel. Are there any ways to do it?
I've found a way how to do it. There are two methods:
While in the UV Editor window, hold down Ctrl, then press the Middle Mouse Button and move the mouse horizontally or vertically. You may now manually fit the image to the borders of the window.
While in the UV Editor window, press the F button. An image will automatically fit the window borders perfectly.
In addition to what others have already suggested, if your keyboard has a Home button, you can press that too, which will zoom the image to 100% size (if it's smaller than the panel) or to the size of the panel (if it's larger).