There's a trick to prevent this behavior:

1) Select the pop-up image.
2) Click the "Link" button in the top toolbar.
3) Move the cursor to the very end of the "Script" field. You should see the following code:

... href="#">

4) Change this code so it is as follows:

... href="javascript:void(0);">

5) Click OK then preview the site - it shouldn't jump back to the top of the page when you click the image.