Make a picture transparant in PPT 2013

There is no such thing as picture transparency in PPT 2013 and previous versions. Here's a workaround.

Place your picture on the slide in the appropriate dimensions.
Draw a rectangle that overlaps your picture.

The shape covers the picture completely once you have released the mouse button.

Select the shape, right click it and choose Format Shape.

Choose Picture or texture fill, click File and select your picture, finally click Insert.

Change the transparancy using the slider:

All that remains is to remove the original picture which is behind the shape. Click on your slide so that nothing is selected. Press the tab key, when you see the Format tab under Picture tools you can delete the picture. (alternative: use the selection pane).

Original picture - Transparent picture

Attention
In PPT 365 you now can make a picture transparent. Microsoft added a tansparency button on the Picture tools format tab. If you click the transparency button you will be presented with presets from 15 to 95 %.

If you have PPT 2019 this is still not available.