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If I understood you correctly, you want to add image you provided as background image of your form. You can add it inside of Form Designer.
You can also add it with CSS.
background-image : url('http://your-image.com/here.png');
background-size : 100% 100% !important;
background-repeat : no-repeat;
background-attachment : fixed;
Replace http://your-image.com/here.png with URL of your image.
Hope this will help. Let us know if you need further assistance.