If you have uploaded an image, but the size is not what you want, you can manually edit the image size. To do that, hover over the image with your mouse and click on the wrench icon that appears in the upper left corner of the image.
The image properties pop-up window appears, where you can change the width and height of the picture (1). It's better to add either only the width or only the height of the picture, this will ensure that the picture proportions will remain the same. If you add both, the picture will be cut to the selected size.
After you have entered the width/height, click "OK" (2) and the image will obtain the desired size.