Many cloud storage platforms can be used as a source for images in a gallery block. However, it is important to check with the specific platform to see if it is compatible with the gallery block. The most popular ones are iCloud and Google Photos. I use Google Photos and https://fastupload.io/en. They both have free storage for unlimited photos if you're willing to let them compress the photos slightly. You can just enter the site and see if it suits your needs. I have used these two platforms since 2018 and have never had any problems.