I'm thinking about building an app that lets users upload images to sites like Flickr. They would need to enter their password to do this. I would send the data through PHP, do I need to get a SSL certificate for that? 
Do I need an SSL Certificate?
security;https;security certificate
You need a certificate if you will use HTTPS encrypting the traffic. Since you are requiring a login I would say that it will be necessary (not technically but sending the information in clear is not a good option).