We create image screenshots using the standard compression for the selected image format, however this can be overridden by changing the quality parameter when taking a screenshot. Perhaps to reduce the file size of the returned screenshot.
Furthermore some image formats produce better quality results than others. Therefore for higher quality images avoid JPG and instead choose another format such as PNG.
Another potential quality issue can be caused by scaling the screenshot. This means increasing or decreasing the image size relative to the size of the browser. While we use the best scaling algorithms the quality can still be effected, this is often due to the developer choosing incorrect scaling ratios for the image.