In the Landing Page Designer, when you select an element in the page, in the upper right-hand corner, is a drop-down that shows the name of that page element. If you click on it, it shows the properties of that element. Select your text element, and check the "Layer" value, the default value is 15. Change the value to a higher number than the image's layer, 20 or so. The text should now be on top of the image.