Your site will automatically scale and crop many images that you upload. In order to prevent this, you will have to crop them yourself before you upload them. You can do this using any image editing software: Gimp, Photoshop, MS Paint, etc. In order to find what dimensions your image is being cropped to, you can download from your site by right clicking it and selecting “Save Image” or “View Image” (or perhaps another command depending on your operating system). You can then view that image’s dimensions in your OS’s explorer window or using your image editing software.
This is a bit technically advanced for a lot of users, so it will generally be easier for us to look up the dimensions ourselves. In your web browser right click on their image and click “View image info” or “View image". If you view the image info, it will show the images dimensions. Otherwise, the image’s name should be something like someband.com/sites/someband.bombplates.com/files/slider_900x200/slides/image.jpg.
That 900x200 is the dimensions. If the band hasn’t uploaded an image, you can upload one yourself by logging into their site as BPSupport. Don’t forget to delete it afterwards!