It wasn't my suggestion originally, I was just letting people know where to find the screenshots. But, as Stone noted, there are ways to convert the file type, such as apps and online sites. You can also reduce the file size the same way. There are many apps and sites that will do that for you with no loss of quality. One little app I use is called Fileminimizer.
Something else to be aware of. While the PNG format will have a larger file size, it will also have a higher quality because it's a lossless format, while JPG is a lossy format.