Well, you’re out of luck then. There is no way to bind an executable to an image without preserving the extension. If you change the extension your file won’t be interpreted as an executable anymore, so any instruction inside the file will be treated as binary data instead, unless you manage to find a vulnerability in an image viewer software, a vulnerability that can make the program interpret part of a specially crafted image as executable code, but it’s unlikely to happen, also you’ll have to hope the person you’re targeting has the right program and version installed. Another very, very unlikely option is finding a flaw in a specific file format, usually these exploits take advantage of a badly implemented feature, that can lead to unwanted behavior. But these are extremely rare cases.
In short, you can’t have a legit image that opens a binded program.