Hi,
You have ot know that 24MB is very very few if you want to process images.
Changing the thumbnail size means : reprocessing the images to generate the new thumbnails.
It is something which require some memory because it has to load the image (uncompressed in the server memory), generate the thubmnail and save the new image.
Depending the size of your images, it can use some memory.
But once the thumbnails ready, it won't consume this memory anymore (except if you add new images).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.