There are few or no ready-to-use C code examples on the Web about implementation of (secure) file transfer protocols with the ESP8266 (ftp, ssh, etc ...) working as client. Most of them cover the HTTP so that based on these source-codes as a template for your project, you would certainly send this via POST command, but on the file server-side you should create a dedicated Web application to get the pictures on raw text and restore to its original binary format.