JPEG is better suited to photos. Sledger is right, a PNG is better for getting cleaner colors such as red which is always a problem, especially with JPEGs. And even in Photoshop.

If the image if for a website then you can probably use 8-bit PNG (256 colors).

But the image is only going to be as good as your screen capture.

Are you creating the images or downloading them from the web? Because if you are creating the charts then it is best if you can export them from the application in which the chart is created.