As background, a jpg file is the very same file as a jpeg file, except that it is simply named with a different (shortened) file extension. This only matters to some programs using the file, not to the file or what's in it. Originally, the DOS 8.3 file-name structure could only support 3-letter filename extensions; newer OS's support 4-letter. JPEG stands for the Joint Photographic Experts Group who developed the "lossy" file format, but that was shortened to JPG in the old days to support the DOS naming standard.
If you can save a jpeg file as a jpg file, most software (but not all, especially with a Mac) will recognize and use it perfectly, just as if it was named as a jpeg file. Certain newer software may specifically seek the jpeg extension, since the longer file extensions are now becoming more commonly used. The best thing to do is try it out and see what happens. As to why Opera doesn't offer to save in jpeg format is for its developers to say.