When you insert your pictures into powerpoint....

try zooming out (view menu > zoom) until you can see all your picture

Then click your picture - then, while holding down the left mouse button,
drag any of the 'corner handles' into the middle of the slide.. You will see
your picture get smaller..... let go when it's just how you want it.

One extra tip - hold down the CTRL key on your keyboard. This makes
powerpoint resize the picture towards the middle.

cheers
TAJ Simmons
Microsoft Powerpoint MVP

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Nice tip re ctrl key!

As well as what TAJ said, PowerPoint doesn't "magnify pictures", it shows
them at the size they are. You might want to try resizing your pictures, you
can do this in batches using the free and excellent irfanview.

http://www.irfanview.com/