Reply To: Tumblr photoset permalink

[TheDoc]

No need for JS here at all.

Change your block to:

{block:Photoset}
    <div class="entry type-photoset">
    {block:Photos}
        <a href="{Permalink}" class="photo">
            <img alt="" src="{PhotoURL-250}">
        </a>
    {/block:Photos}
    </div>
{/block:Photoset}

Then in your CSS:

.type-photoset .photo {
    display: none;
}

.type-photoset .photo:first-child {
    display: block;
}

This will give you browser support all the way down to IE 7.