I'm using the theme Manifest and would like to make my blog wider to accomodate bigger pictures

Note that a better way to add CSS to a WordPress.com blog if you are new to CSS is to select the "Add my CSS" instead of the "Don't use Manifest's CSS" option in the CSS Settings on the Appearance → Custom Design → CSS page.

Try updating that setting, removing all of the CSS you have now, and then add just this to the editor on the Appearance → Custom Design → CSS page and save changes:

#site-wrapper,
#main-nav ul ul,
#core-content,
#comments h3,
legend span,
#footer,
.footer-content {
	width: 900px;
}

If you wanted to keep the widgets on the right side similar to how they're showing up now, you might try something like this instead:

#site-wrapper {
	width: 1000px;
}
#core-content {
	width: 700px;
}
#footer,
.footer-content {
	float: left;
	padding-left: 30px;
	width: 270px;
	margin-top: -15px;
	padding-top: 0;
	border-top: none;
}

You can remove the "Recent Posts" by removing it from the "Footer area one" container on the Appearance → Widgets page.

Try the changes above and please post back here with an update about what other spacing you're trying to change.

[IMG]http://i45.tinypic.com/11ux7pf.jpg[/IMG]