A couple of things I can think of could be happening here.
Firstly, to view the bigger version of your photo, place your cursor over the centre of the picture on the item discription page, a box should appear in the centre and the larger picture should appear to the left, by moving your curser around you will be able to move around the larger picture to see greater detail.
If this doesn't happen it is in indication that your picture is not high enough in resolution to enlarge further on the screen. I think from memory your pictures need to be 500 pixels at least on the shortest side for an enlargement to become active.
To adjust the image so it has higher resolution you may need to adjust the size in an image editing program such as gimp, photoshop etc
