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The featured image should be square – ideally 300 px.

The featured image is used for displaying the thumbnail on the main page. This image needs to be square so the rollover effect can slice it in half properly. 300 pixels in width and 300 pixels in height is the recommended size. An image 400 pixels in width and 300 pixels in height will display oddly, but 400 pixels x 400 pixels would be okay.

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There are a few options for the portfolio which can be found in Theme Options > Styling Options. Portfolio Item Rollover Color – the background color of the feature image.
Portfolio Single Background Color – the background color of the portfolio single popup.
Portfolio Single Navigation Color – the background color of the portfolio single post navigation bar