The key is the resolution at which you've stored them on your hard drive. If you've stored them at a resolution equal to or greater than the screen's resolution, they will display as full screen images. (You're probably looking at 1,024 x 768 pixels minimum).
I use a free image editor called IrfanView. In it, I have the ability to find out information about the image - including resolution/number of pixels etc. Your image editor probably has the same facility.
If you have stored your pictures at a lower resolution in order to make the files smaller for emailing, then the only way of making them large again is to resize them - again, using an image editor. For example, you can resize an image from 300 x 200 pixels to 600 x 400 or 1200 x 800.
(I just checked: MS Paint has this facility too).
However: if you have stored an image at a low resolution, increasing the resolution will generally result in a loss of image quality - the more you enlarge it, the more the image quality will deteriorate.
What resolution are your images stored at?