Issues with picture and image transforms
You might try putting putting something like:

&lt;IfModule mod_php5.c&gt;
  php_value max_execution_time "30" 
  php_value memory_limit "256M"
&lt;/IfModule&gt; 


inside your .htaccess. file. 

You can tell what your PHP values are right inside Craft... unless you changed your admin trigger, just visit http://server/admin/utils/phpinfo and search for memory_limit and max_execution_time. That'll give you some clues as to what's going on.

Some hosts set these values really low but the default execution time is 30 and memory limit is 128M.

If you run PHP as a cgi, some hosts allow you to use ini files in lieu of .htaccess. Typically these are at the root of your home folder but they could be stored elsewhere. Put memory_limit = 256M in one and throw it up in your home folder. Running php_info() would tell you the location of the current in use one.