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JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Unfortunately, this widget is placed inside the iFrame container which is why you can't apply custom css just by injecting it.
In order to make the image to appear in the center, you can reduce the width of the form using the Form Designer tool. Under the Form Layout, put 550 on the form width. You also need to enable the "Make this form responsive" option.
You also need to apply this custom CSS code for the text above the widget to appear at the center of the form. In the form designer, go to the CSS tab then paste this code.
text-align: center !important;
width: 100% !important;
Another way of injecting custom CSS: http://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes
Hope this helps. Let us know if you need any help.