When you upload and image in Weebly, the image perfect editor automatically crops your image. So the image quality directly relates to the size of monitor you use. If you are using a larger monitor, you can upload a larger and higher resolution image. However, if you have a smaller monitor, Weebly restrict the size of the image base on your monitor size. No matter what image resolution you use, if your monitor is small, Weebly will convert that image to a smaller resolution image. We have tutorial in our wiki that will show you some tricks to upload the header image: http://divtagtutorials.com/why-does-my-full-width-header-image-look-different-in-different-monitor-size/.