Image formats for websites
The formats used are JPG (JPEG), PNG and Gif.
In general, JPGs are used for photos while Gifs are used for graphics such as logos; and animations. PNGs, which can also display images with transparency effects, can be used for both photos and transparent images.
A digital image is a collection of pixels (dots) and each pixel is told what color and brightness to be to create the image - so more pixels equals more data and a bigger file size. An 8 megapixel camera will produce files of around 2mb, but the image will also be around 3000 pixels wide; around three times the width of a typical website.
The mistake many people make is uploading a full size picture to their website then changing the size in the article editor - the problem is, it's only when the full size picture arrives in the website visitors browser that the size is changed - the browser has had to download the full size image in order to make it smaller, which is a bad thing to do.
If the page has maybe one large image, it shouldn't be too noticeable - start to do the same with five, ten or more images and things are going to get very slow...
I hope this will be very useful to those who don't know about images for websites.