Set z-index of fixed elements

Hi, I am trying to set the Z-index of a vector element through javascript (I can change the element to PNG if absolutely necessary)
The element in question is this whale:


The idea here is that I want the whale to disappear behind the next section as I scroll down. That’s why I made it “fixed”. Although when I get to another image in the next section the whale is on top of the image.
Is there a way to set the z-index of the whale (and the image) so that it goes behind the image lower in the page?

Thanks

You can set layer by clicking with the right mouse onto your image and then search for “order” or “arrange” or somethng like that.

The z-index on page follows the sequence on Layer>
The Layer (tools => Layers) shows a element tree.
Using “send to back” or “bring to front” changes the position on that tree. you can use drag and drop to rearrange.
keep on mind, the tree sequence should be the loading order.
The elements out of sort will generate the page scrolls down/up on the loading.