Hi,
I know that it is not the perfect answer but I can't tell you why the image is not processed correctly.
That is linked with the PHP GD library and I don't think that we can do something to fix it in HikaShop.
But I can recommend you to use PNG files instead of GIF files. The compression is generally better and you can also have the support of the transparency ; plus you can have a better palette of colors.
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.