Please have a detailed read of the NewFile documentation. NewFile does not create a new file, not at all! It sets up a Go os.File which wraps around an existing file with the given file descriptor (0 in your case which is stdin I think).
Serving images without files is much easier: Just Encode your image to your ResponseWriter. That's what interfaces are there for. No need to write to ome magic "in memory file", no need to read it back with ReadAll, plain and simple: Write to your response.