Yeah like I said if there is a large image it will popup the problem I saw is all the images your uploading are like 450x325 or so which means they are all under the medium threshold of 600 you have set so there is no large image the main image is what your uploading.
Here is an example so you understand DSC_0082a - My Photo Gallery
Look at the picture you uploaded it is like 319x480 pixels. Therefore there is no medium image so there can be no large image to popup. Your photo you uploaded is the only image there except the thumbnail.
How things work are like this. You upload a photo most people nowadays that photo may be 3264x2448 or larger if taken with a digital camera.
So what happens if if you upload an image the image will be resized down to the max dimensions you specify in upload options which for you is 1024 either width or height. Now if the image is also over the medium threshold for you this is 600 a medium is created.
Therefore based on your thresholds the right images are created. So if you continue to upload photos smaller than 600 pixels the only thing your going to see if that image and a thumbnail.