PNG is really high resolution image.

No, PNG is a format that retains all of the required pixel information about an image without degrading it - independent of resolution.

JPG just drops some graphic and colors which we hardly see.

No, JPG is a lossy format that tries to reduce file size without compromising the subjective quality of the image. It loses information in the image and in doing so adds 'artifacts' that degrade the picture quality. The more the jpeg encoder compresses the original image, the more picture quality is compromised. This format is particularly suitable for images that are photographics and with less fine detail that needs to be preserved - unsuitable for solid areas of the same colour or preserving fine detail.