You can't override resources using properties from Form Builder. (You can override oxf.xforms.… properties by editing the source, adding an attribute on the xforms:model, but here you need to override an oxf.fr.… property.)
And at this point, you can't override the image used in the submit button, or other similar buttons, with a property. For this, you need to edit the buttons.xsl file (look for the template for fr:submit-button).
Alex
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