To rescale the images without losing quality, you want to start with images that are larger than the HD frame size, so the you can reduce the scale to have them fit, as opposed to enlarging them (which will introduce artifacts as you scale them up).
Open your ppt document in Power Point, then go to the menu File > Save As Pictures
When the Save Dialog opens, choose the format you want the files saved in, in this instance JPEG, and click the Options button:
When the Option Pane opens, set the dimensions of the frame to something slightly larger than the 1920 x 1080 HD frame size of your FCP timeline, and make sure Save every slide as a series of graphic files in selected, then click OK.
Then click Save.
Now import those frames into FCP, and when you use them they should be scalable to the size you want without too much noticable degradation. You may need to render the stills to see them properly, depending on the horsepower of your computer.