<?php
$html = 'Muita cena aqui pelo meio e com tags<br><img alt="imagem 1" src="imagem1.jpg"/><br/><img alt="imagem 2" src="imagem2.jpg"/><br/>';
preg_match('@<img.+src="(.*)".*>@Uims', $html, $matches);
echo $matches[1];
PD9waHAKCiRodG1sID0gJ011aXRhIGNlbmEgYXF1aSBwZWxvIG1laW8gZSBjb20gdGFnczxicj48aW1nIGFsdD0iaW1hZ2VtIDEiIHNyYz0iaW1hZ2VtMS5qcGciLz48YnIvPjxpbWcgYWx0PSJpbWFnZW0gMiIgc3JjPSJpbWFnZW0yLmpwZyIvPjxici8+JzsKCnByZWdfbWF0Y2goJ0A8aW1nLitzcmM9IiguKikiLio+QFVpbXMnLCAkaHRtbCwgJG1hdGNoZXMpOwoKZWNobyAkbWF0Y2hlc1sxXTs=