Ive tried this  and it repeats the last image twice.

 {exp:channel_images:images entry_id="{entry_id}" limit="5"}

        {if image:count == '1'}
             <a href="{image:url}" rel="shadowbox[{entry_id}]" title="{title}"><img src="{image:url:small}" alt="{title}" /></a>

        {if:elseif  image:count != '1'}
              <a href="{image:url}" rel="shadowbox[{entry_id}]" title="{title}" id="myhidden"> <img src="{image:url}"  alt="{title}"  />
          {/if}
     {/exp:channel_images:images}

And ive  tried this  which repeats the first image twice and the last image twice

{exp:channel_images:images entry_id="{entry_id}" cover_only="yes"}
         <a href="{image:url}" rel="shadowbox[{entry_id}]" title="{title}"><img src="{image:url:small}" alt="{title}" /></a>
    {/exp:channel_images:images}

{exp:channel_images:images entry_id="{entry_id}" skip_cover="yes"}
        <a href="{image:url}" rel="shadowbox[{entry_id}]" title="{title}" id="myhidden"> <img src="{image:url}"  alt="{title}"  />
    {/exp:channel_images:images}