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!