The Save as... is the right way. When saving, make sure to pick .jpg (or .png if you have transparent image) under the file name (the Files of type box). The actual file format is controlled by a box below that, but it is changed automatically.
Then there is the matter of file extension. It should be also changed automatically, but only if you have not already changed.
If you have .jpg in Files of type and the file name, does not have .jpg extension, rename it.