That's because you specify the filename absolutely:
String fileName = String.format("wallpaper.jpg" ,System.currentTimeMillis());
File outFile = new File(fileDir,fileName);
Unless you specifically change that file name, every file you download will go to the same file, overwriting all the others.
You might want to think about how you want to store these: different folders for each day or week maybe, date and time stamped filenames, incrementing numbers, ... there are a lot of options you can pick from.