The phpThumb class has issues with static functions as you could see on
http://www.otkup.hr/image.php?src=assets/images/antikviteti/IMG_0537.JPG&h=318&w=450&zc=1&hash=ee4b6acb136fb0e081c311e87d585488
But this is caused by image.php and not by the phpThumb class itself. The image.php does not belong to the phpthumb core package in Evolution 1.0.15. The phpthumb core snippet will create a full path to the thumbnail image in assets/cache/whatever on your server.
You will have to change your PHx based phpthumb calls to direct phpthumb snippet calls i.e. from [+image:phpthumb=`w=150,h=76,far=C,bg=FFFFFF`+]
to [[phpthumb? &input=`[+image+]` &options=`w=150,h=76,far=C,bg=FFFFFF`]] to solve that issue.