Currently in a Photo Contest with more than 50 images, the "gallery" view is split into multiple pages. When someone opens up the "large image" view, and scrolls through to vote on each image, it only loops through the images on that specific page. So for example, if there are 60 images, it splits into 2 pages of 50 and 10, then in large view starting from image 1 it scrolls through only to image 50 and forces the voter to click back to Gallery and Next Page.
You are right, that's a shortcoming of our current image viewer.
We are working on a better image viewer, where this is fixed.
We will notify you here once it is released.
We plan to release it within the next weeks.