Welcome to the world of WordPress where at first it's tough but it definitely gets easier with time. You'll be pleased to know that there are a handful of plugins that exist that give you the granular control of where and when widgets are displayed. For example, you might have one sidebar in a theme to house widgets but with plugins such as Widget Logic, you can use conditional statements that can allow one widget to show up on the front page while hiding it and showing a different one on the Single page view.
Try http://wordpress.org/extend/plugins/widget-logic/ but also look at http://codex.wordpress.org/Conditional_Tags if you're thinking about using it.
for something that is a bit more user friendly with the same ideas in mind, try http://wordpress.org/extend/plugins/widget-context/
As for centering images, I've always had luck of using the Center HTML tag within the Text Widget but I learned the best way was to have a class within the themes style sheet for centering images and then encasing the text or image within the Text widget with that class.
For example, I have the following within my themes Style sheet
Code:
.center, .aligncenter {
text-align: center;
} And the following image within the text widget.
Code:
<div class="aligncenter"><a href="http://feeds2.feedburner.com/WordpressTavern"><img src="http://www.wptavern.com/images/subscriptionicons/bluerss.png" alt="Subscribe Via RSS" title="Add WPTavern To Your FeedReader So You Never Miss A Beat"></a>
Hope that helps you out.