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Image processing during the upload

It often happens the situation when a user uploads photos directly from the camera. Such images have a big size and significantly slow down site performance.
Enable images auto resizing - Quantum Manager
Enable images auto resizing
Automatically image resizing - after enabling this param the image will be resized according to the width and height which you need to set up in settings.
But keep in mind that uploading big files and processing them require more resources. Therefore allocate more memory for scripts executing in .htaccess file and also specify a maximum upload file size.

How to configure?

For Apache server you can apply the following rules to .htaccess file:
php_value memory_limit 256M
php_value upload_max_filesize 50M
php_value post_max_size 50M
You can change these limits personally according to your requirements and server resource.
Additional tutorial: How to check PHP memory limit for Joomla​