Hi Isaac, 

You should be able to save an image/coupon in Android fairly easy with a button and the right attributes. 

However, doing so in iOS is not possible at the moment. Due to the way mobile Safari handles certain attributes and sandboxing of apps, the user needs to explicitly trigger the saving prompt.  

You can see a list of what browsers support the download attribute right here. 

Based on your target audience, you might have to show a screenshot/instructions on how to save an image (long press/hold on an image to trigger the saving prompt).

Best,

Hristian