How accelerators for dial up compress images 

The way accelerators for dial up work is by compressing web page images before they reach your computer using a technique known as lossy compression. Once the images reach your computer they are recompressed.  Recompressed images are similar, but not exactly the same as they were originally. Much of the digital detail that the human eye can't detect is removed from web page images.

As a result it appears you are downloading these images faster. In reality there is much  less data in the image to download. Look at the above dial up compression examples compared to the same uncompressed image. Can you see the difference?

The compressed image on the right was displayed on my computer monitor nearly 3x sooner than the non compressed image on the left without compromising  image quality. All images on a web page are compressed using the same method. Time saved waiting for web pages to load will add up quickly.