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