Outdated TYPO3 Version
This documentation refers to an outdated TYPO3 version - either select a supported version or make sure to use a TYPO3 Extended Long Term Support (ELTS) version to continue getting security updates.
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!