If you replace in a new created responsive Form and change the image size to another value at example to 15% for width and height the results ar only viewed on the Designer and an exported PDF File. The changes will not have any effects on the Form if self.
The HTML code of the Form view:
<div class="nf-filler-control nf-filler-highlightonvalidationerror nf-image" style='background-image: url("https://xxx.yyy/SiteAssets/Image.png");' data-formcontroltypeid="c0a89c70-0781-4bd4-8623-f73675005e08" data-controlid="8e2cc5ce-d509-40e7-b96c-97dd1c2299cd" data-uniqueid="c0a89c70-0781-4bd4-8623-f73675005e08" data-controlname="xxx.yyy" formcontrolid="8e2cc5ce-d509-40e7-b96c-97dd1c2299cd"><img class=" nf-image" id="ctl00_ctl33_g_8059ab36_100e_410b_bbe1_7f82556e7f0e_ctl00_ListForm2_formFiller_
FormView_ctl03_ctl01_ctl02_f1024f4b_1fd0_473e_867a_4f7047088444" style="width: 15%; height: 15%;" alt="xxx.yyy" src="https://xxx.yyy/SiteAssets/IMAGE.png"></div>
Found this issue on CSS:
At the CSS file responsive-common.css
(https://xxx.yyy/_layouts/15/NintexForms/responsive/assets/styles/responsive-common.css?v=4.3.2.0)
On the Line 598 the property background-size is set to cover, but should be set to contain to resize the image for complete view!
The correct css code should be as in the following image:
