[TYPO3-english] tifs on server do convert to jpg when click-enlarged

[Matthias Stief]

ahoy,
my photos lay as tif-files on the server.
when shown in galeries, they are fine as jpgs, but when clicked and enlarged they are converted into gif-files and I have no clue why.
See here: matthiasstief.com/testsite/   
TYPO3 version 7.6.21. 

Maybe someone has a hint? 

best regards, Matthias

www.matthiasstief.com