Its actually a native windows thing, binary combining.
Open up command prompt on the folder containing your files (.rar and image file, I've used .gif and .jpg)
I like using the dir command so I get the file exactly right.
Type:
copy /b file1.jpg+file2.rar file3.jpg
of course using the real file names, and file3 can be named anything so long as its extension is that of the image file.