On magical scroller app setting page, under "Upload Loader Image" section you can see Choose File option. Just click on this and then upload the loader image that you want on your website (Only GIF image is allowed). After that Click on "Save Settings" button. Then new uploaded loader image will display on your website collection page.

Yes. Magical Scroller is fully free. There are no hidden charges.

To uninstall app, click on "Apps" on the left hand side of your shopify admin then you will see "Magical Scroller" app in your Apps list.

Click on the Delete icon to uninstall the app.

No, you don't need to add any code manually. Everything is done programmatically.