1. To centre the "View Image" link, you can give it a class, then target in your stylesheet and applydisplay:block; text-align:center;
2. The font-size is a side-effect of your website not being responsive. Amongst other things, your site isn't using a<meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1" />metatag, and you are using explicit pixel units when sizing your fonts, which don't translate so well when adapting to different devices/viewports (or rather, they *don't* adapt), so the "Close Window" text will remain the same as the rest of your text unless you increase it manually. If you'd like to try that, give it a class, then target in your stylesheet with a much bigger font-size. BUT, this will lead to vastly disproportionate font-sizes when compared to the other text on your site, so what I suggest doing instead, is making the whole of the modal overlay/window into a "click to close" trigger. To do that, give the "Close Window" link a class, then target it in your stylesheet and apply these additional styles;width:100%; height:100%; display:block; position:absolute;
BTW, the modal overlay is picking up some top and bottom margin overrides from default div styling that exists elsewhere in your stylesheet, so you might want to addmargin:0;to the .show div CSS to make it sit flush with the top and bottom of the screen.
3. Superseded by number 2?
4. Yep, use your own tile image (for IE7/8) if you wish. You will also need to change the RGB background values against the .show div CSS to match. As a starting point, trybackground:rgb(255,204,204); background:rgba(255,204,204,.75);. (.75 is the degree of opacity).

