You need to optimize/scale the image in an image editor like Photoshop. The source of the problem is with the image size itself. In Photoshop you want to change the image size to the actual true image dimensions. ie if you are adding the image to a container that is 1000 x 300 then your image size should also be 1000 x 300. Then you want to save it as a progressive jpg. If you do not have Photoshop you can send the image to us and we will optimize/scale/resize it for you. I need to see the actual website page where the image currently is so send that link/URL with your image file.
“…but I wasn’t too thrilled about the loss of image quality/detail after publishing”. Yep, you are going to lose quality. That is one thing about the whole “flat” website design trend that makes sense to me. It is more important that your website loads as fast as possible vs “showing off”.