Uhhh, "Create thumbnails" have to be turned off... As nothing happened when changed the thumbnail sizes I started playing with the thumbnail cache settings to see the result and set it to zero (0) that was a fault.
Still nothing changed so just found a promising "Create thumbnails" option and turned it ON. That was my second fault. I am not sure about which cache (K2, or GK... or there is one more(!) cache settings in this module at the Basic settings) processed my commands... lets say 30 minutes later but all the images disappeared.
Almost a day later just created a fresh Gk5 news module and realized that there "Create thumbnails" is turned off and thumbnail cache is set to 30 mins. Reproduced and voilรก everything get better not in 30 mins but immediately.
NIGHTMARE.