IMO this is actually a really good decision given the purpose of the service. A lot of my older family members that I will likely invite to my new account (great idea btw) are familiar with sending pictures through SMS and don't really use any other messaging apps or many apps at all, if their phone even supports apps. Believe it or not many older people and some younger people still rock low powered phones with little or no data plan. However most if not all phones made within the last decade can send SMS messages and more than likely are on a plan with free texting.
Why should I need to download another app that is a glorified REST client when SMS does the job perfectly well?