How to send an image over Java HTTP server

You're doing way too much work here: decoding the image, and storing it in memory. You shouldn't try to read the file as an image. That is useless. All the browser needs is the bytes that are in the image file. So you should simply send the bytes in the image file as is:

File file = new File("/root/images/test.gif");
arg0.sendResponseHeaders(200, file.length());
// TODO set the Content-Type header to image/gif 

OutputStream outputStream=arg0.getResponseBody();
Files.copy(file.toPath(), outputStream);
outputStream.close();