How do I change a photo file extension from htm to jpg?

I was trying to upload a picture on another web site but it said that it was an invalid file extension. My pics usually are jpg or jpeg. I don't know how this happened. It says it is a Firefox file and I open it with Microsoft.How do I change a photo file extension from htm to jpg?
htm and jpg are a little like apples and oranges. There is no real simple way to change htm to jpg. The only way I can think of is to open the htm file then do a screen capture by pressing FUNCTION Prt Scrn, then save that as a JPG image.How do I change a photo file extension from htm to jpg?
You don't.. HTM is the language used to create webpages. JPG files are graphic files, they are two very different things.