How to decrease picture size

you will need to add this css to a child theme or install a custom css plugin

.blackwell_top_image {
    height:400px; //change this to whatever height you want the image to be
    overflow:hidden;
}

It has to be done this way as the image width is set to 100% so setting a height on the image itself would distort the image. So we are adding the css to the parent container and hiding the rest of the image.