You can adjust the transparency of your background image within your form's Theme window.
Navigate to the droplet icon in the top-right of the form editor (Theme), and select UI Elements → Background.
Below the box labeled "Background Image", locate the field "Overlay Color" under the section titled "Overlay".
Click on the box provided and a color picker window will appear.
Near the bottom of the color picker you'll see a rainbow bar and a checkered bar - the checkered bar is the one that controls the transparency of your image.
You can adjust the background transparency by dragging the slider bar left and right, or by entering the value manually (a number between 0-255) in the field labeled "A" just below the transparency slider.
You can also add overlay blur to the image in the same theme section. Play around to achieve the desired look of your form's background!
This option is also located in the "Overlay" section of "Background".