The effect you are seeing may be lessened by reducing the resolution of your image (and Adobe Flash Player will not have to scale it down as much as it currently does to fit into the gallery's image area).
Alternatively, you could change the quality Flash parameter (as documented here). By default, a Flash presentation will use the 'high' quality setting. You could force SimpleViewer to use the 'best' quality setting as follows.
(1) Open the following Showkase template page (depending on which theme you use) in a plain text editor:

\showkase\_themes\boma\pagetypes\simpleviewer.tpl
\showkase\_themes\kosel\pagetypes\simpleviewer.tpl
\showkase\_themes\zero\pagetypes\simpleviewer.tpl

Search the file for:

var params = {};

... create a new line beneath this code and enter:

params.quality="best";

Now go to 'Pages -> Edit Gallery Page -> Advanced -> Rebuild this SimpleViewer Gallery'.

2. How can I set a separation line between gallery and footer?

You could edit the same page as above and add <hr> before the opening <footer> tag.

Thank you for reporting. I have notified the developers of this problem and it should hopefully be fixed in the next version of Showkase.

I do not see this shrinking effect when I view your gallery in Firefox 21, Chrome 27, IE10 or Safari 5.1.7.
Hovering over a link on your web page should not affect the display of any images within a Flash presentation (such as your SimpleViewer gallery) on the same page.
What browser(s) do you see this effect in?

No. Showkase was not designed with such functionality in mind (to allow users to upload their own thumbnails to the '\showkase\_data\thumbcache\' folder). The best solution would probably be to keep your own custom thumbnails in a separate location and replace the thumbnails in the '\showkase\_data/thumbcache\' folder with your own as and when required.

When you said "How can I set a separation line between gallery and footer?" I thought you wanted it only on your gallery pages. If you want a horizontal line above the footer in all pages, add it to the top of your footer by going to 'Site -> Site Settings' and changing:

© copyright

... to:

<hr>© copyright

... in the 'Page Footer' text field.

My suggestion uses a standard HTML <hr> tag.
You could enclose your footer in a div and style the div with a top border by entering the following code into the 'Page Footer' text field:

<div style="border-top: 1px solid #333333; height: 50px;"><br>© copyright</div>

Different browsers use different rescaling algorithms and this may result in images looking slightly different cross-browser.
Try resizing your logo image (it is currently 586px x 292px) before feeing it to Showkase so that it will be displayed closer to its actual size and browsers do not have to rescale it as much as they currently do.

Sorry. I thought you were talking about the links in the navigation menu. I see the slight change in image size when hovering over the thumbnail images on a Gallery Index page in Chrome 27 and Firefox 21 and have notified the developers of this.

To increase the width of the content column on an 'About' page in the Kosel theme, open the \showkase\_themes\kosel\css\styles.css file in a plain text editor and increase the width and max-width values on line 601-602.
To do the same for a 'Basic' page, change the max-width value on line 551 of the same file.
Please note that the line numbers refer to the current version of Showkase (v1.0.4).

Edit:
Alternatively, rather than modify the styles.css file, you could add the required CSS (below) to the \showkase\_themes\kosel\css\custom.css file. Any CSS in the custom.css file will take precedence over the styles.css file.

body.type-basic section.content p,
body.type-basic section.content h1,
body.type-basic section.content h2,
body.type-basic section.content h3,
body.type-basic section.content h4,
body.type-basic section.content h5,
body.type-basic section.content h6,
body.type-basic section.content ul,
body.type-basic section.content ol,
body.type-basic section.content dl {
    max-width: 560px;
}

div.profile {
    width: 55%;
    max-width: 560px;
}