PowerPoint doesn't have a tool that automatically makes an entire image transparent. To apply this effect, insert a shape in the slide to hold the image and then apply transparency to it. However, you can make one color in an image transparent without using a shape as a placeholder.
Make an Image Transparent
Open the Insert tab and select Shapes. Choose a shape that matches the shape of the image, such as a rectangle, and select it.
Hold your mouse over the slide -- the cursor shows as a cross -- and click and drag to insert the shape. Don't worry about the size and position of the shape right now; you can edit it later.
Right-click the shape and select Format Shape.
Select Fill and then Picture or texture fill.
To add an image from your computer, select File to locate the file and insert it. Use Clipboard to paste an image that you've copied to your system's clipboard or select Online to search for images in Bing Image Search or a OneDrive account.
Resize the image by selecting the squares on its corners or lines and dragging them in or out.
Move the Transparency slider to increase the image's transparency or use the arrows in the Transparency percentage box. The effect previews on the image as you increase the percentage; stop when you get the desired level of transparency. Select the X on the Format Picture box to close the menu and apply the change.
Make One Color Transparent
You don't need to add a shape to hold an image if you only want to make certain parts of the picture, defined by their color, transparent; just insert the image in a slide.
Select the image and open the Picture Tools tab. Select the Color button in the Adjust area and then Set Transparent Color. The mouse cursor changes to a pen shape. Click the color you want to make transparent.