How to remove border and background from my image submit button?

You would change the style just like any other element in your theme by adding some CSS to your theme stylesheet to override the styles your theme is applying globally to this element.

Edit:
http://bangkokhomecondo.com/wp-content/themes/agentpress/red.css

Add this to the bottom of that CSS file:

.gform_footer input {background-color: #fff!important; border: 0!important;}