There is zero difference between a file named
.jpg
.jpeg
.JPG
.JPEG
If your graphics program can output a file named .jpeg, you can rename that file .jpg
The file is literally identical, differing only in name.
The renaming to .jpg is recommended due to a weird glitch in 8ch back-end software that has been reported to CM and will likely be fixed sooner or later.
.PNG files tend to be 6x larger than equivalent .jpg files. Memefarmer converts most graphics to .jpg with a 93% quality setting before archiving, to save space in the archive.