To upload an image into your Presentation.

Go to Presentations (top bar)> click on the presentation to edit.

In the list at the left,  add a slide or click on the slide to be modified.   In order to add a new slide you can navigate to How to add a page to a presentation 

Select Image within the chosen slide format.

Select the file to upload.

The file must in the one of these formats: .png or .jpg.

Use the Edit menu (bottom of screen after clicking on the image) to replace the image or delete it.

For information on presentations, see this article; Getting started with Presentations: How to create a pack for a meeting; How to create and edit a presentation