Make sure that you've enabled editing on the document, before getting started:
Once in PowerPoint with the file open:
Click where you want to insert the picture on the slide.
On the Insert tab, in the Images group, click Pictures, then click This Device.
In the dialog box that opens, browse to the picture that you want to insert, click that picture, and then click Insert.
For more pointers on editing in PowerPoint, check out our PowerPoint Tutorial in Tech Tools and Training!