image formats
When it comes to image formats on the internet, it's generally a three-way tie between JPEG [http://en.wikipedia.org/wiki/JPEG], GIF [http://en.wikipedia.org/wiki/GIF], and PNG [http://en.wikipedia.org/wiki/PNG]. Deciding which image format to use is relatively straightforward; you choose
image compression
It's surprising that the venerable JPEG image compression standard, which dates back to 1986, is still the best we can do for photographic image compression. I can't remember when I encountered my first JPEG image, but JPEG didn't appear to enter practical use until
image compression
Over the years, I've standardized on a JPEG compression factor of 15; I find that generally provides the best compromise between image quality and file size for most photographic images. Although I've done some ad-hoc testing that pointed to compression factor 15 as the sweet spot