In Powerpoint open an image.
In the image on the left you can see that the text is hidden behind the image.
In case image has a white background, the best solution to remove the background is by using the following procedure.
Double click on the image to open the Picture tools tab.
Click Set Transparent Color.
Now click on the background of the image.
As you can see on the image on the left, the white background was removed and text behind the image is now visible.
In case image is using more than one color for the background, you will have to use different procedure.
Again double click the image to open Format tools.
PowerPoint will automatically remove background.
In case you are not satisfied with the result, you can use Mark Areas to keep and Mark Areas to Remove to improve the result.
On the left side you can see another example of the multicolored image where background was removed.