Please do the following steps: (This example created based on Apex 5.x)
Upload an image in the share component
Go the property of the region that you would like to set background image (Region Property > Advanced-> Static ID)
Set a Static ID of the region ( in my case i have set Static ID = NOMINATEFORM) and save it.
Now go the property of the Page (Page property > HTML header)
In the HTMl header section type the below style, just copy and paste below:
6. In the above style command you need to change the image path on your way.
7. Now Apply changes and run the application. you will see the input form control showing front and the image set as background