Can't upload images from frontend theme

Hey guys,
I'm working on a theme and I've found out that uploading images doesn't work on the frontend, only at the backend via wp-admin.

Checked my error-log and found this error:
"Aq_Resize.process() error: Unable to resize image because image_resize_dimensions() failed"

What should I do now? Is there a fix? Where? In theme or server?

Can't find the right solution on Google.

Thank You!