500 Internal server error for generated icons/img

Hi,

I hope someone can help me with the following problem:

https://www.familydynamics.com.hk/development/
This project that I am working on is currently in beta.

While the theme is newly installed with codes to generate icons, sliding banner and stuff,
no image is showing up. The images I’ve uploaded and inserted to a post are showing up with no problem. But on homepage, the images which the theme should pull from the post are not showing up, as well as the icons generated by the theme.

When I try to open the broken icons (right-click, open image in tab), it shows a generic 500 Internal error.

I wonder if this is about permissions. But I have checked and everything is under 755, which should be right, correct?

p.s. Hope the chinese doesnt bother anyone.

Any help is very much appreciated!
Thanks!