It depends on weather or not you have the original pictures, or if the pictures are just ones you have uploaded from somewhere else. (if that makes any sense)
Actually, I don't know if that makes any diff, but if they are not from your originals, then they may just blurr out when you enlarge them.
Open the picture in Microsoft Office Picture Mangager and select Edit picture, then select to resize the image in the pop up box on the right side. Save the image and then upload it. It should appear bigger now.

Thanks! and GL!