Thank you for your answer! It kinda works, but it was a bit tricky to get done. This is how I managed to do it in the end:
I set the property "background-image" to "url(images/myimage.png)" -> the images are saved in the images folder after export, not in the Files folder (as I first thought).
BUT the sprites are combined on a sprite sheet. In order to display just one specific file, I had to put the image into the image folder in the zip file AFTER exporting it.
By the way, the form buttons don't have the opacity option.
But anyways, it works now, so thanks again!