The query string format for GET/POST requests is key-value pairs like key=value, separated by &, with + replacing any spaces.
parameter1=value1¶meter2=value2¶meter3=value+number+3
It looks like you were trying to use jQuery syntax. In jQuery you can pass an object { parameter1: value1, parameter2: value2 } which jQuery will convert into a query string like above.
And then the base64 encoded image could contain & or + or other characters that could be confused as part of the query string so you need to wrap it in URLEncode().