By default, Event Gallery uses the original image for download and sharing options. You can restrict this by opening the file /components/com_eventgallery/config.php and modify the constant COM_EVENTGALLERY_FLICKR_MAX_IMAGE_SIZE to a value of 2048, 1600 or 1024. To keep this change even after an update, follow the instructions on top of the file and create the override file config_override.php.

This change has only impact for the regular frontend. If you sell those images, people will continue getting the original images with their purchase.