Every month. W3Techs records the percentage of websites using popular image file formats. For a long time, PNG was slowly creeping up on JPEG. In the latest numbers, PNG has pulled ahead, being used on 74.1% of websites, against JPEG’s 73.8%. GIF stands at a distant third with 36%, followed by SVG at 3.8%.
Keep in mind, this isn’t the same as saying more PNG images are on the Web than JPEG images. If pages which hold JPEGs contain more of them, there may be more JPEG images even if they aren’t on more sites. Since galleries of JPEG photographs are common, this is a plausible situation.
You can see the trend by looking at the RSS feed. Unfortunately, all links point to the same URL, but if you can view the RSS preview, you can get the old numbers on a monthly basis. The web page is updated daily, which is why the numbers quoted above don’t match the June numbers in the RSS feed.
GIF, in spite of all its hype, is shrinking steadily in usage, while SVG’s gain is dramatic in relative terms.
As storage and bandwidth grow cheaper, there’s less need for lossy compression. This is very likely the reason more sites are using PNG images.