TYPO3 v8 has reached its end-of-life March 31st, 2020 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
web (gif or jpg ..)
Image extensions can be anything among the allowed types defined in the global variable $TYPO3_CONF_VARS['GFX']['imagefile_ext']. Standard is pdf, gif, jpg, jpeg, tif, bmp, ai, pcx, tga, png.
The value "web" is special. This will just ensure that an image is converted to a web image format (gif or jpg) if it happens not to be already!