Hi,
The original image is uploaded correctly in the server but the displayed image is a resized image.
It seams that you configure the shop to resize images with a 600px max width.
If your images are 600px width, you can access by FTP to your website and copy the original files to the "thumbnails" directory.
HikaShop use the PHP GD Library to making thumbnails, it is possible to set the compression level but we didn't but an HikaShop option for it.
I think that do not use the 600px thumbnails would be better because you can compress files with more settings than a simple compression level.
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.