are you editing the S2 layer code directly? or are you putting this in the customization wizard somewhere?

if directly, then okay, for example, in the S2 layer I am using, the part of EntryPage::print_comment() that prints the userpic is:

if ($*opt_userpic_comments and defined $e.userpic) {
    """<div class="user-icon">$e.userpic</div>""";
}

The stuff inside the """ is a string that's added to the outputted html, with ; marking the end of that expression

so you could try instead replacing it (on yours) with

"""<a href="http://$e.poster.username""" + """.dreamwidth.org/icons">""";

or,

"""<a href="http://$e.poster.username""";
""".dreamwidth.org/icons">""";

(should be effectively the same thing and if the former doesn't work you can try the latter; I recently had a weird issue with something not concatenating like I expected it to)

if you have a double quotation mark in your string then you put it inside triple-quotation marks like that.

if you are doing it through the customization wizard instead somehow then you could ~try~ putting <a href="http://$e.poster.username""" + """.dreamwidth.org/icons"> in the field, that mayyyyy work? but you may have to edit the layer S2 code directly.