Quality doesn't have to be print quality or aesthetic, etc. just enough to show the point you are making.
There are lots of image examples attached in this topic, so you can see the file type used, the sizes involved and does the quality of the image convey the information you want.
I now pretty much use .png format not (on my screen capture software) set at 8bit 256 colours and that is more than adequate for web page display of this sort of information without having very large file sizes. Previously I used .gif at a slightly reduced quality, I did all of this whilst still being locked into dial-up and trying to keep size down.