You need to be in timeline mode, I think by default the program in in what is called storyboard mode, so you see the image or video in a frame surrounded by functions.... if you look to the left of the first frame image then you will see three button icons, the middle one is most likely the one that is selected, so you need to press the bottom icon and you will switch to timeline.

In timeline mode you can select (so it is highlighted) the image you want to delete and delete it by pressing the delete button on your keyboard. you are not left with just the text there, so you can find the image you want to replace it with and drag and drop it into that location.