NextGen looks great in the feature area.  One way you can do it is just add a little bit of CSS to the style.php file that actually just spreads the images out across the feature area.  If you are using a consistent number of images, like 10 in the screenshot, then just add something like this in the style.php:

#feature img { margin-right:3px; }

Then after adding that at the very end of the style.php file, save it, go into your theme settings and click a "Save changes" button. That will rewrite the stylesheet.

Preview your blog and see if the images space correctly.  You'll need to adjust the '3px' value to get it spaced correctly, just try 2 or 4 or whatever works.  As long as you keep 10 images at the same thumbnail size it will look good.

Hey, it looks great!  Very classy look.  I even like how you styled the footer by removing the background color but left the footer feature area in to create a gap between the content and the footer.  Personally, I think the calendar looks good that way.  It really goes well with the rest of the blog.

Nice work!

And that was the point of creating the theme, just to give the average blogger the ability to tweak their own theme as they wish. There are imitators out there now, but there's only one Flexibility!

You are correct on the style.php.  The padding between posts and sidebar is a little tricky because the widths of both are calculated in the style.php file based on your theme settings for the content width.  You'll see calculations like this:  $sidebar = $flex_blog_width – $flex_content_width – 15;

It's really kind of complicated to explain all the styles used to set the padding and margins, but if you have Firefox, you can download the Web Developer add-on and that allows you to see which CSS element is currently highlighted. Then you can look in the style.php file for the appropriate styles.