Note that when you change that directory, you will no longer be taken to your screenshots when you tap the pop-up notification that says, “screenshot taken, tap to view…”, because the Gallery app looks in the Screenshots directory for them. You can fix that by opening /usr/share/jolla-gallery/Gallery.qml, and find the lines that say something like “file path… standard path picture +” screenshot"", and change it to file path “/run/media/nemo/(sdcard label)/”.
Maybe @coderus can add the option to ScreenTapShot app…