[Mooff]

And now we know it's not fopen or url_access.
Resizing and everything works (not gonna look too good though given the width/height ratio of the pictures) but the slider can't save the pictures (cause the folder either does not exist or isn't writeable).

change:
//your slider folder - has to be read and writeable
//folder where all slider files are saved
$slider_img_filepath = 'Forum/slider/';
to

$slider_img_filepath = 'slider/'