If you have a website that has a space along the top for a banner then you will need to think carefully about the dimensions and aspect ratio of the banner when it comes to choosing the right photos. If the banner space is log and thin then there is no point trying to use a photo that is almost square as the majority of the image at the top or bottom will have to be cropped off.
If you are struggling to find good landscape images that you could use then you could buy one from stock photos such as from Istock or Getty images. Buying them will allow you the right to use them on your website but does mean other websites may have the same image.
Another way you can get round the odd size of your image is to have an abstract image that focuses on something zoomed in. This will allow you to be a lot more creative when it comes to the design.
If you want banners to scroll on your website make sure that you allow people to read the text (if there is any) before the images scrolls on to the next one and giving the customer the option to scroll back and forwards through the banners is often a good idea.