screen shot - how to turn PNG into Jpeg

What good is a PNG anyway?

To answer the last question first, PNG files are higher quality than jpgs and support transparency.

If you press "command + Shift + 5", click on options, you can send screenshots to Preview automatically, then edit/save the screen shot in any format you like.

This MacRumors article provides information on how to change the default screenshot file format (in addition to some other things) that may help.

https://www.macrumors.com/how-to/save-location-format-macos-screenshots/

Once you have the screen shot created you can open the Preview App and change the file type. No need to use Elements. You have Preview pre-installed on your system, you can easily find it using Spotlight.

In the Terminal, and prior to Mojave, you would enter the following to change the default image format with the screencapture shortcuts. By default, Mojave does not include the key 'type' in its screencapture plist, and one must add it with the appropriate image capture format.

Prior to Mojave (as tested on High Sierra) — because I had set it to png:

defaults read com.apple.screencapture

{
    "disable-shadow" = 1;
    type = png;
}

defaults write com.apple.screencapture type -string jpg

defaults read com.apple.screencapture

{
    "disable-shadow" = 1;
    type = jpg;
}

Now your screen capture shortcuts will output JPG images.

Mojave and later

Make a backup copy of the screencapture plist file, and then add the new key type with its string value jpg.

cp ~/Library/Preferences/com.apple.screencapture.plist ~/Library/Preferences/com.apple.screencapture.plist.bak

defaults write com.apple.screencapture type -string jpg

defaults read com.apple.screencapture

{
    "last-messagetrace-stamp" = "568258083.601015";
    "last-selection" =     {
        Height = 231;
        Width = 380;
        X = 286;
        Y = "369.5";
    };
    "last-selection-display" = 0;
    "location-last" = "/Applications";
    style = selection;
    target = file;
    type = jpg;
    video = 0;
}

Now, on Mojave, all screen capture shortcuts (including the new ⇧⌘5) will default to JPG image captures.

If you want to change the JPG back to PNG again:

defaults write com.apple.screencapture type -string png

The article that iCare linked you to lets you permanently select the format and save location of the screenshot. No conversion necessary.