As per the Stamp card images best practice guidance it is important that you do not use an image with a transparent background for the Icon image.
The following images illustrate how different background colors will look on the iPhone lockscreen.
Here's a logo with a transparent background:
This is what this logo looks like on the iPhone lockscreen. Because the iPhone converts the transparency layer to black for the lockscreen and the logo, in this case, is predominantly black you can't see the logo. You just see a black square! That's not what you want your customers to see. You want them to see your brand and your icon.
So, here's a logo without a transparent background. If you are not sure how to save an image without a transparent background, a quick and easy way is to save the image in JPG format. The JPG image format does not allow transparent backgrounds.
And what it looks like on the iPhone lockscreen.
And lastly here's a logo with a colored background.
And what this looks like on the iphone lockscreen.
So, in summary, if you want you, your card and your brand to look its best on the iPhone lockscreen do not use an image with a transparent background as your loyalty card icon image.