Featured image error on Network front page

Hi guys,

I'm in the process of converting a well-established and high traffic site to Wordpress to convert it into a membership site. I have chosen the Network theme, since that is one of the objectives of the new site (creating a network of sub-sites for regions/sectors) and have re-created a blog post from the existing site.

This post appears on the front page, but when I try to add a featured image to replace the theme default image, I get the following error:

Warning: preg_replace(): Unknown modifier 'a' in /home/acg/public_html/wp-content/themes/network/library/functions/custom-functions.php on line 695

I even tried changing the image size to the Network theme "standard image" size (which would be annoying if I had to create an extra image for each post instead of resizing/cropping an existing image), but to no avail. The only way I can avoid the error while not displaying the picture of cyclists and a message telling me to add an image to the post is to add a default image in Advanced Options and delete the featured image.

That's the way it's currently set up on the test site - but clearly I need to know why the error is occurring and solve it so I can display the featured image on the front page blog post summary.

Thanks in advance for your help.