This banner is a JPEG (or jpg)
A JPEG is one of the two kinds of bitmap image files most widely used on the net and in digital cameras and camera phones. Another old and very common kind are GIFs.
JPEGs and GIFs are readable by Macs, PCs and even ancient browsers held together with bits of string. They're readable by tablets and smartphones. JPEGs and GIFs are about as universal as you can get.
There's another advantage to being as common as mud. Jpeg and gif images are unlikely to become unreadable by tomorrow's technology. An important point to remember if you are saving digital pictures for posterity.
If treated properly, gifs and jpegs can also make very small files.
JPEG today is the most common format in which digital cameras and camera phones save photos.
Gifs have the advantage that they can include animation. Gifs also allow transparency, so cutout shapes can stand out against the background colour of a page. Jpegs permit greater subtlety of colour and shading.
JPEG images give wonderful effects, but they are 'lossy'. They are widely used because they make much smaller files than most photo formats. They do this by removing picture elements the eye can't detect. Beware! Edit a photo and save it once and your picture looks none the worse. Titivate and save it several times over and you will see what you've lost.
During another comparison test - of art materials - I painted an orange with oils and scanned it into my computer. I converted the picture to a JPEG (shown first) and a GIF.
You can see that the JPEG conveys the nuances of shading.
The GIF curdles and posterises the colour.
I had to reduce the .gif to only eight colours in order to cut down the file size. Even then it's 6,136 bytes - almost half as much again as the JPEG at only 4,261 bytes.
Unlike the example above, many.gif pictures can be reduced to extremely small files. You have to experiment to get the best results. Automated processes can help you so far, and no further. Photoshop allows you to cut down the colours of a GIF to an absolute minimum, with a good deal of control. Put your rainbow on a ration with Photoshop or Elements and you can make really small files.
They say GIFs are not 'lossy'. That is, they don't lose information every time you save them as JPEGs do. Don't forget, though, that if you remove colours from a .GIF, then save it, the colours are lost.
ALWAYS KEEP YOUR ORIGINAL before you make changes. Make a copy, edit that, and save your edited versions with different names.