That’s a difficult question to answer without more information because depending on how you configure Wordpress and how many plugins you are using it can be quite memory intensive. I would certainly recommend at least a 2GB droplet to start and then you can check your CPU load and see if you need to resize up.

Since you are running a stock photo website you are also processing a lot of static image requests so I would recommend that you run nginx as your webserver. Regardless of whether you use DigitalOcean or not this should drastically improve your performance, because nginx can serve those static file requests directly while running Wordpress with Apache will actually create each one of your Apache instances with a lot of memory usage which will degrade your performance.

So if you deploy Wordpress with nginx as your webserver you will get a lot more capacity on any provider that you are running on.