If I understand you correctly, you are downloading the image as a base64 encoded string, then passing that string into an img src? If that is the case it sounds like the scope of your strings is not expiring somehow. To ensure the deletion of a primitive object like a string, you must keep a reference to the string by proxy with an object. Something like below.
// Somewhere in a persistent scope like global
image_reference = {};
// In your ajax callback
if(image_reference.data != null) {
delete image_reference.data;
}
image_reference['data'] = response.data;
// Put image_reference.data into your img element