To add a logo to your form, open the Form Editor within the form builder and click the Form Header. Click on the plus sign and select Image. A lightbox will appear where you can upload the logo image.
To make sure that your logo is also mobile responsive, after you upload the image, in the Dimensions section, type 100% in the first box (width) and auto in the second one (height). Disable the Constrain proportions option to maintain the original width and height dimensions.