Text Form fields to show on top of Image Form Field

It's not a good idea to have overlapping fields since you can't explicitly control their z-order (ie, which one will appear on top of another). It's basically works so that the last field you edit (not fill it, but actually manipulate in Form Edit mode) is the one on top, which is not very convenient. I would strongly recommend looking for an alternate way of doing it.

You could try using the button label.  Image form fields are actually buttons. And it can be setup to show a label over the button image. Look at the Options tab on the field properties. Set the Layout to one of the Label plus Icon options. 

You'll need a way to get user entered text onto the label. I'd suggest adding this code to the existing MouseUp script on the "image" (button) field.

var cLabel = app.response("Enter Label Text");

event.target.buttonSetCaption(cLabel);