<div id="mainimage">
<mt:Assets type="image" tag="@SITE_MAIN_IMAGE" limit="1">
<img alt="<$mt:BlogName encode_html="1"$>" src="<$mt:AssetThumbnailURL encode_html="1">">
<mt:Else>
<img alt="" src="<$mt:SupportDirectoryURL encode_html="1"$>theme_static/eiger/images/mainimage-sample.png">
</mt:Assets>
</div>

メインイメージサイズ⇒960*300px

tag="@SITE_MAIN_IMAGE"