#!/usr/bin/perl

while(<DATA>) {

    if(m/<img\s+src="\/images\/blank.gif"/ && !m/display:block/) {
        print;
        s|(<img\s+src="/images/blank.gif".*?)>|$1 style="display:block">|;
        print;
    }

}
__DATA__ 
<img src="/images/blank.gif" border="0">
<img src="/images/blank.gif" border="0" style="display:block" >
<img src="/images/blank.gif" border="0">