To blow up pictures on power point to fill the screen

I am about to submit a portfolio to my employer with the collection of single images in low resolution, watermarked and compile a PPS PowerPoint presentation.  I find the latter to be more professional as you can arrange pictures according to order, one slide with headlines to subdivide pictures according to groups.  You can even refrain from the watermarking as pictures are embedded and easier to handle.  But how to blow up pictures on power point to fill the screen?  This might be more than 100% if the image resolutions are below the screen resolution and it results soft. How to avoid picture deterioration?  Any recommendations from graphic designers? Thank you very much for hearing my problem.