Large images and memory limit

Issue Summary

Hello,

I am currently finalising a website offering the users to upload large high definition images (300dpi, .jpg, max 35Mo), but this seems to be tricky.

An error message keeps showing on certain node editing pages when trying to save :

Fatal error : Out of memory (allocated 220463104) (tried to allocate 28348 bytes) in [chemin du serveur]/www/new/sites/all/modules/imageapi/imageapi_gd.module on line 59

In the file sites/default/settings.php I have added these lines (following the various advises here and there. But the values I entered might not be optimal) :

ini_set(‘memory_limit’, ‘512M’) ;
ini_set(‘post_max_size’, ‘256M’) ;
ini_set(‘upload_max_filesize’, ‘128M’) ; [I also tried with 32M and 64M, but no difference]
ini_set(‘zend.ze1_compatibility_mode’, 0) ;

What do you think?
Are my memory_limit parameters ok? Or does this have something to do with an imageapi bug?
How should one proceed in order to upload and download large images through Drupal 6?

Technical information :
Drupal 6 / PHP 5.2.17 / my hosting compagny does not allow me to access php.ini

Thank you for your kind help!
Clément