<?php
if(!defined("B_PROLOG_INCLUDED") || B_PROLOG_INCLUDED!==true)die();
$thumbWidth = 300;
$thumbHeight = 200;

foreach($arResult["ITEMS"] as $key => $arItem) {
	if (is_array($arItem["PREVIEW_PICTURE"])) {
		$arFileTmp = CFile::ResizeImageGet(
			$arItem["PREVIEW_PICTURE"],
			array("width" => $thumbWidth, "height" => $thumbHeight),
			BX_RESIZE_IMAGE_EXACT,    // или BX_RESIZE_IMAGE_PROPORTIONAL
			true
		);

		$arResult["ITEMS"][$key]["THUMB"] = array(
			"SRC" => $arFileTmp["src"],
			'WIDTH' => $arFileTmp["width"],
			'HEIGHT' => $arFileTmp["height"],
		);
	}
}