Hello,
There is no feature to change the orientation of text (or other content) within the attach serial plugin.
The GD lib would allow that (an angle parameter) but it would make the configuration screen even more complicated and I'm not sure that mirroring is supported.
So that's not something I can recommand.
If you can modify your image template to make it horizontal, it would be better for the plugin and your customer who want to see it's coupon image.
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.