Amazon has been incentivizing Prime members to use Amazon Photos as their cloud storage service of choice with occasionally offering gifts of on-site credit when they upload their first picture or video. The company is taking this Prime Day (and the week following) to push another promo, offering $15 if you download the Amazon Photos app and give it a try.
Google Photos discontinued free storage for lightly-compressed photos and videos at the start of the month, so it's fairly smart of Amazon to gin up some help for its service which allows unlimited full-resolution still backups — there's a 5GB cap on video which can be expanded with a subscription fee. There are plenty of smart cases to be made for Google, but if you need raw storage and have Prime, there's no beating this.
Remember, this offer is for first-time Amazon Photos users only and media must be uploaded by 11:59 p.m. PDT on June 30. Within 7 days, customers will receive a $15 credit on an Amazon cart worth at least $30 containing products that are sold by Amazon.com or Amazon Digital Services LLC. Even if an item from a third party is being "fulfilled by Amazon.com" or is "Prime Eligible," they will not count towards the threshold. The credit must be redeemed by the end of the day July 15.
Amazon might have already sent you an email about this offer if you're eligible, but if you tend to ignore those messages or even have them to skip over to spam, you can check this page to see if you're eligible.