Create a floating image

Step by step

Add a white background.
For the "horizon", insert a square blue shape.
Resize it so it stretches over the horizon.
Right-click, select Edit object settings, and go to the Transparency tab.
Here, select Use gradient transparency and click down the down arrow in the transparency direction tool.

Lock the position of the shape (Right-click > Lock > Position > of selected objects).
Insert the image.
Duplicate (CTRL+D) and flip (from the General tab in the Object settings) the image.
Place the flipped image at a distance below with the horizon inbetween.
Right-click the flipped image, select Edit object settings, and go to the Transparency tab.
Here, select Use gradient transparency and click down the down arrow in the transparency direction tool.
Set the transparency to 60.

Select both images. Right-click and select Movement and then Advanced movement.
In the Timing tab, select In and out move and enter 0:10 in the In duration and Out duration text boxes.
In the Position tab, select Border and Right under From position and Current under To position.
Under X-curve, select the bottom predefined curve.

Now it's time to add the "floating" movement. Right-click the upper image and select Movement > Multiple movement > Add > Advanced movement.
In the Timing tab, select Single move and enter 0:10 in the Start time and 20:00 in the Stop time text boxes.
In the Position tab, select Current under From position and Relative under To position.
We want it to move subtly up and down, so enter a value say 10 in the Y box.
Under Y-curve, select the top predefined curve.

For the flipped image, repeat the last five steps but enter -10 in the Y box, since we want the mirror movement as well.