Index: wp-admin/js/theme.js
===================================================================
--- wp-admin/js/theme.js	(revision 26734)
+++ wp-admin/js/theme.js	(working copy)
@@ -250,6 +250,8 @@
 		this.activeTheme();
 		// Set up navigation events
 		this.navigation();
+		// Checks screenshot size
+		this.screenshotCheck( this.$el );
 	},
 
 	// Adds a class to the currently active theme
@@ -352,6 +354,23 @@
 	previousTheme: function() {
 		var self = this;
 		self.trigger( 'theme:previous', self.model.cid );
+	},
+
+	// Checks if the theme screenshot is the old 300px width version
+	// and adds a corresponding class if it's true
+	screenshotCheck: function( el ) {
+		var width;
+		// Wait until it's loaded...
+		el.find( '.screenshot img' ).on( 'load', function() {
+			width = el.find( '.screenshot img' ).css( 'width', 'auto' ).width();
+
+			if ( width <= 300 ) {
+			 	el.addClass( 'small-screenshot' );
+			}
+
+			// Restore CSS value for width
+			el.find( '.screenshot img' ).css( 'width', '100%' );
+		});
 	}
 });
 
Index: wp-admin/css/wp-admin.css
===================================================================
--- wp-admin/css/wp-admin.css	(revision 26734)
+++ wp-admin/css/wp-admin.css	(working copy)
@@ -6894,6 +6894,15 @@
 	top: 0;
 	width: 100%;
 }
+/* Handles old 300px screenshots */
+.theme-overlay.small-screenshot .theme-screenshots {
+	position: absolute;
+	width: 302px;
+}
+.theme-overlay.small-screenshot .theme-info {
+	margin-left: 350px;
+	width: auto;
+}
 
 /* Other screenshots, shown small and square */
 .theme-overlay .screenshot.thumb {
@@ -7162,6 +7171,15 @@
 	.theme-browser .theme .theme-actions {
 		padding: 5px 10px 4px 10px;
 	}
+	.theme-overlay.small-screenshot .theme-screenshots {
+		position: static;
+		float: none;
+		max-width: 302px;
+	}
+	.theme-overlay.small-screenshot .theme-info {
+		margin-left: 0;
+		width: auto;
+	}
 }
 
 @media only screen and (max-width: 480px) {