Your first use case seems perfect for Dropbox... just share a link to the specific files in question. The second use case, I'm unaware of anything that legally

Your first use case seems perfect for Dropbox... just share a link to the specific files in question. The second use case, I'm unaware of anything that legally provides that guarantee (i.e. with an SLA), but if you trust that Amazon will be around for 10 years, you could just sign up for an AWS account and put the images on S3 yourself. I'd put more stock (literally and figuratively) in Amazon than Archive.org for data reliability.