This is caused by a script trying to use more memory than can be allocated to it. By default on shared servers, any single php script can only consume 8Mb of memory. This is a hard limit set in the php.ini file and cannot be changed for stability reasons.
This problem happens when using scripts that dynamically resize large images into smaller versions. Image resizing is an extremely memory hungry process so running into this error is very likely. A workaround is to pre-process the images and store them in another location or cache, or you can call on another script written in a language that does not have this memory limit problem, e.g. Perl, to do the actual processing.
You can also add a .htaccess file with the following command to increase the php_memory_limit:
php_value memory_limit 16M