You may zoom into your photos in Photosmith for iPad just like you do on iPad's native Photos App. Using two fingers, pull in opposite directions (“un-pinch”) to zoom the photo in. Depending on the size of the image, you may need to do this several times.
To zoom out, simply “pinch” your fingers together.
Alternatively, you can double tap on the image to zoom to 100%, or double tap to zoom back out to fit-to-screen.