no ability to use the php internal mail sending.
It seems that the plugin doesn't use the cpg mail function, but the PHP function mail
OK, the above topic can be ignored.
Please post your solution.
a) Is it possible to add more text to the mail a user gets after downloading an image?
Have a look at the file fullsize_secure.php
and check the mail functions. I guess you either have to adjust that line:
$txt = $CONFIG['plugin_ks_fullsize_message_for_customer'] . "\n";
or adjust the corresponding config value.
b) Is there a way to insert the Album-Name and the user name as a variable?
Should be possible by adjusting the some code near the code you found in a).
c) For the access rights of the full sized image: The "secured" images are getting the rights 660. In the settings of coppermine there is an option to set the default access rights of files. When setting 660 there, does this save the step from securing images via the plug in?
If Coppermine has sufficient permissions to change that, I guess you won't be able to access any generated files (thumbnails, intermediate-sized versions). I haven't checked that nor looked at the code. I suggest to test this on your gallery.