The file was not found or the script has not enough rights to access the
file.
Check the file name and path. Check user and file permissions.
NULL
"Could not open file_name"
The file is already opened and exclusivly locked by another application.
In the most cases a programm opens the file for writing.
addHTMLImage() does no file locking, so this problem is
not caused by competitve callings of this function.