Unfortunately, it is all to common to see improper image formats used in web and blog pages. Even people who should know better are stuck in a JPEG rut, when JPEG encoding severely degrades the quality of the image. In Export Chart as Image File and Image File Type Comparison of Exported Charts I have tried to explain why PNG is a better image format for Excel charts.
JPG is for photographic images, and for art with continuously varying color gradients. PNG is for line art, screen shots, text; in short, for most anything computer-related. Even the ancient GIF format is usually better than JPG.
Thanks to Karl Peterson for the link.