[Mooff]

Some ideas:

• set the fallback image switches to true:
  $show_articles_without_image = true;
  $fallback_img = 'external/unitedforum/slider/uf_fallback_slider.png'; //path to an existing picture
  
  If the images show up we at least know the slider is working correctly and the bug is somewhere in the vb4 implementation
  
• the images are resized using the php GD package (vbulletin does support both GD and ImageMagick) - the slider currently has no ImageMagick support. Do you have the GD package installed?
  
• comment out the first and the last line
  

  //ob_start();
  
  //ob_end_clean();

  You will then see error messages and warnings (given there are some), once you load the cms page where your slider is active. Please send me this output.