Can I use a picture as a form background in Legacy editor?
The following article is about legacy forms.
Legacy forms are no longer developed and are not available in accounts
Any image in your gallery can be used as a web form
To add a background image, open your web form in the editing screen. On the right side, choose the Style tab and move to the Input section. Click the Background>>Image tab and click the icon to open your gallery. Choose the image you want, adjust the tiling option, set the scale, and voilà!
In the section where you see existing images in your gallery, you can add more pictures by clicking Upload from URL