Large images consumes much bandwidth, slow down the page loading, use more storage space so since a long time Facebook using a smart technique for the photo uploading process. When you upload your photos of any supported formats by them, Facebook first upload it and optimize the image. It actually convert the image from any other image format to optimized JPG image format. So, as we all know optimized JPG format consumes less storage and losslessly optimizes them, without decreasing images quality but size.
But unfortunately, sometimes JPG just behave strange and lowers down an image quality and even if you have a very high quality images (PNG format) then the beautiful, high quality high resolution image turns into a disaster.
As million's of photos are uploading regularly, Facebook knows if they let users to upload high quality images like PNG then it will increase load to their servers, use high bandwidth, increase its loading and consume much resources and storage units so Facebook is not about change it anyway.
So, you might be uploaded a high quality photo, but it turns into a disaster and couldn't find a solution that how you will upload your favorite image, profile pic, cover photo of high quality PNG format or many be any other format. But, you can do that by using a simple trick.
How to add High Quality Photos On Facebook
Facebook will not allow you to upload your high quality images by its default uploading process.But you can use third party services to easily upload your favorite photos in high-quality and those images will be in any format eg: PNG.
The apps that provides similar services are HootSuite, BufferApp and others. These apps allow you to pre-schedule Facebook posts and with Photos. However, most of the social posting services are premium ones. So, it won't allow you to use the service for free.
BufferApp is a free one, and i would like to recommend this app to you. I always use BufferApp to schedule and post photos on my Facebook Timeline.
As per the free plan by BufferApp it will allow you to schedule up-to 10 posts for Facebook profile or page, but unlimited instant status and image posting.