并没有做定语后置！！这就是普通的动词短语high up，句子拆分我认为是这样的:those (who thought that ( the images higher up the attractiveness

the images were higher up the attractiveness scale
higher，形容词
up the attractiveness scale 介词短语

应该就是三楼的说法，吸引力更高的照片，higher up the attractiveness scale做定语修饰名词image，很好造同样的句子：The man taller than him is his elder brother，比他高的男人是他哥哥

不是high up的形容词固定搭配，就是简单的high的比较级+介词短语

those 主 who... 定从
corres.. 动词
that images... thought 的宾从

images (that are) higher up...