Follow these steps to add a foreground image to your hero/cover section:
Open a case study in edit mode.
Hover over your hero/cover section.
Click Add foreground image on the right side of the section.
The foreground image will be initially inserted as a mockup, but you can turn off the mockup by following these steps:
Hover over the mockup.
Hover over the Mockup button at its bottom.
Choose No Mockup.