I realize that this is an old thread, but I was just working on it and needed a reminder myself (since I already set this up for my company a few years ago, but needed to do it again recently).
There is no need to use a separate tool, you can make it work using the "Picture fill" function in Powerpoint. Basically you need to:
- Edit the picture you want to insert, and adjust the canvas size so it has adequate empty space on the right side (can also add more empty space on the bottom, it'll make it easier in the next steps)
- In the Powerpoint template, select View > Slide Master
- Insert a normal rectangles where you want the picture, and set border to "none"
- Right-click > Format picture > Fill
- Select "Picture or texture fill", and choose the picture you want
- Check the checkbox "Tile picture as texture"
- Change Scale X/Y to 100% (or to another percentage if you need)
The rectangle has to be big enough that it works both in 16:9 and 4:3 scale, since the rectangle size will change when you switch scale. That's why you need to adjust the canvas size on the image file so that the right side of the image will just be white (or transparent), even if the rectangle is wider than the actual image.
PS: If the image needs to be right-aligned, just add the empty space on the left instead, and select "Alignment: Top right" after step 6.
For background pictures you can also check "Tile picture as texture" to make it either tile or be cropped when you switch the scale. You might then have to adjust Scale X/Y manually to make the background image fill out or shrink to the correct size.
Edit: Apparently this causes various issues, amongst others the images will be broken if someone tries to use "compress all images" in their PowerPoint, so it's not actually a very good solution after all. Also some PPT previewing web sites won't display it correctly.
Leaving it for anyone curious, but after testing it out for a while, I do not recommend this solution for actual use.