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You should be using alt tags, not H1 tags. You should only have one H1 tag per page that describes what the page is about (ideally containing the keyword you are trying to rank for). You probably shouldn't be putting any text behind images as this may be treated as cloaking. If you really want to keep the text behind the images, wrap it in the <noscript>tags so that it is only visible when javascript is disabled.</p> <p>Other possible solutions is to overlay (caption style) images with your desired text in H2 tags.</p> <p>Cheers</p></noscript>

You really should leave the extension alone for this exact reason - when something gets borked - you will have issues. Making friendly URL's, but like John said, you don't need to alter the extension.