In order to get this to work in IE, you'll have to use the <input> tag rather than the <button> tag as far as I know. I don't think that you can get around this.
I'd rather use the thumbnail by preference, the trouble was I couldn't figure out how to make the thumbnail work in the same way as my button is
If you'd like to make your thumbnail the link, then why not just put anchor tags around the image tag?
Code:
echo '<A href="displaypic.php?'.$trimname.'"><IMG src="'.$file.'" alt="'.$file.'"/></A>';
I don't know if that exact code will do what it's supposed to do given your variable names and such, but I think it's a start...Also notice that the style of sending information to displaypic.php is no longer a 'post' style, it's a 'query string' now. Up to you in terms of how you want to design it, but that should make your code work in IE as well as any other browser.
I hope this helps =).