reducing app size

You can compress them any time you want. Just make sure to save the file after compression with the same name

Can you compress jpegs after the app is completed by going into the Interactbuilder file.  Do you use the assets folder in player or the files in the main interactbuilder file?

As long as the new file in the app folder has the same file name as the original file, the app should refer to the updated one. So if you want to take any of your image files through TinyPNG, you can just replace them in your app. When the smaller file is ready in your Downloads folder, change the name of the file to match the original, and move it to your app folder. Your computer will ask if the file should be replaced, and you can click "Yes." The app will then use the new file instead.