most websites use .jpgs and .gifs
.pngs take up too much memory and a good web designer knows this
a website needs to have the important content loaded in at least five seconds or the user will just leave
most .jpgs and .gifs look horrible
a good web designer knows this
GIFs have a color limit and only have 1 bit of alpha (AFAIK?) and JPGs do not support transparency at all, and will look terrible if you compress it enough to make a sizable difference. GIFs should only be used for animated things and JPEGs should only be used for actual photography, never clip-art, logos, or graphics.