The best online Progressive Image generator, free to use, no plugins required.
In simple terms, a progressive image will actually show the entire image right away. The trade-off here being that it only loads some of the image data at a time. This means the image will load in at full size looking pixelated and will become more clear as it loads the entire image, which is more appealing way to deliver an image at modem connection speeds and users with faster connections are not likely to notice the difference because the complete image gets downloaded faster. On the other hand normal images render the image partially, and the browser has to wait until the entire image is finished downloading.
Learn how to create progressive images for your web application