Well, depending on what your web form looks like, you should be able to apply a background image using a similar process to what I described here. Create a div with a background image you want to use and try placing your webform inside it.

Unfortunately, if your autoresponder service’s web forms are designed with a background image or a color, your custom background will get covered up. It might be possible, however, to try and override their style settings with some extremely specific CSS classes:

CSS works on specificity, so if you identify the element of the form that triggers the background design, and then write new styles referencing that element as being nested in your own container div, you should be able to eliminate any background they’ve set up.