[jQuery] Has anyone now how to simplify the following expression by avoiding using attr("id")

[varlo]

Has anyone now how to simplify the following expression by avoiding
using attr("id")

alert($("#"+$(obj).attr("id")+"~p:last").attr("id"));

I need this because not every image could have an id.

<script language="javascript" type="text/javascript">
    function ExpandCollapseOnLeftPanel(obj) {
        if ($(obj).attr("src") == "/Content/Images/arrow_bot.gif") { //
expand
            $(obj).attr("src", "/Content/Images/arrow_side.gif");
            $("#" + $(obj).attr("id") + "~p:last").css("display",
"block");
        }
        else //collapse
        {
            $(obj).attr("src", "/Content/Images/arrow_bot.gif");
            $("#" + $(obj).attr("id") + "~p:last").css("display",
"none");
        }
    }
</script>

<div class="blok">
    <img id="iTA" src="/Content/Images/arrow_bot.gif" alt=" "
class="l_ar" onclick="ExpandCollapseOnLeftPanel(this)" />
    <p id="p1">
        Tracking activities
    </p>
    <p style="display:none">
        <a href="#">home</a><br />
        movie...@lucy,@Bob(3)<br />
        @Canlis table for 2 at ..(1)<br />
    </p>
</div>