Go to Tools>Photo Editor and adjust your image with the Photo Editor tool so it is as close to the size you are wanting.
Try to use images ending in .png or .jpg which are smaller in size.
Avoid using .bmp as the size will be large.
Full width image on content page: 916px wide
Half width image on content page: width="458"
(image below is width="458" by height="343")
Third page = 305px wide
Quarter page = 229px wide
Desktop banner: 1920px wide by 430px tall
Keep in mind the safe area that will show if the window shrinks is within the boundaries of your image.
Tablet banner: 768px wide by 368px tall
Mobile banner: 375px wide by 324 px tall
Button Content Box Icon - 50px wide by 50 px tall
Alert Pop Up Content Box Icon - 60px wide by 60px tall
Image - Rounded corners