ImageMagick is an image manipulation program that runs in the command line.
The most common and easiest use of ImageMagick is converting image file formats.
All you need to do to convert a png image to a jpeg is install ImageMagick, open up a terminal tab, and type in the command below.
convert input.png output.jpeg
The “convert” in that is the base command name, there are also a few others such as identify. “input.png” is the image file that already exists, if you have a different file format you want to convert, just change the file name there. “output.jpeg” is where the converted file will be saved to. If you want it to convert to a different format, just change the file name and extension. There are a lot of other useful tools in ImageMagick, if you want to learn how to use more of them, I suggest reading their docs.