Please follow the below method if you want to change the loader image.
Open the file in this path wp-content/themes/fitnesszone/header.php using some editor like DreamWeaver and scroll to line no: 27 approximately.
Under the loader-wrapper you can change the loader icon.
Note : We have used font awesome icons for it.
Step 1 : Get into the file path /wp-content/themes/fitnesszone/images/ there you need to upload your image.
Step 2 : Then kindly refer the image for how to customize the fontawesome icon to your own custom icon.
Note : You need to change the image name as per your need.