Where does file_get_contents come from? It is a PHP method to open and read a file from the server(!). But image_data is just a string containing the image data as a data URL (base64 encoded).
And "save" doesn't seem to be a parameter, neither is "&title". You have three parameters "image_data", "author" and "title". The & ampersand is just a separator and will be automatically removed from PHP when processing the query string.