If you are using a Mac you don't need any third party software to reduce the size of your photos.
Save them into Photos, edit/crop if you wish, and then click 'file' on the top toolbar and choose export You will be asked for Jpeg quality and size - medium and medium is generally fine for TSP gallery - and then choose where to put the exported file; desktop is easiest.
Then when you insert an image on TSP you get the 'browse' option so navigate to your desktop and choose that image to upload.