Since i could not find any working solution here for my problem with changing the gift card image that is send to customers via email. I thought it would be helpful to drop my solution here. Up till this date. The shopify manual seems incorrect.
I don’t know what version of shopify you have. But if you have this kind of code in your email template:
and you exchange gift-card/card.jpg for your own link to your customized picture in the content library and it does not work. Then you also need to delete this part: | shopify_asset_url
in my case this worked
example of how it should look after changing it:
The picture link in this example is: https://cdn.shopify.com/s/files/1/0704/8026/5480/files/Picture-Giftcard-Engels.png?v=1744186114
I also exchanged it with a picture that was exactly set on 240 160 pxls .. but i dono if that really matters
Goodluck!