Some images not loading after mod install

I just installed the 'multi cross sell' mod and suddenly my website stops displaying most of its images.

The cache shows a debug file containing the following:
Can someone decode this into english for me please?

[21-Sep-2012 01:50:33] PHP Warning: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /home/pazzleco/public_html/admin/includes/functions/extra_functions/reg_multi_xsell.php:41) in /home/pazzleco/public_html/includes/functions/sessions.php on line 113
[21-Sep-2012 01:50:33] PHP Warning: Cannot modify header information - headers already sent by (output started at /home/pazzleco/public_html/admin/includes/functions/extra_functions/reg_multi_xsell.php:41) in /home/pazzleco/public_html/admin/includes/init_includes/init_templates.php on line 28

Note: I've changed my real admin file name to 'admin' for security :D

I cleared my cache and all is working. However I am still concerned as this could happen to my customers who will most likely not clear their cache :/

What could be causing that to happen?

Inxie:

PHP Warning: session_start() [<a href='function.session-start'>function.session-start</a>]: Cannot send session cache limiter - headers already sent (output started at /home/pazzleco/public_html/admin/includes/functions/extra_functions/reg_multi_xsell.php:41) in ...

See the FAQ article on the topic. It can be found by searching in the FAQs for "headers already sent".

thank you sir, much appreciated