How do I restrict the size of my featured image to stay within the left margin?

Hi

I have extended the left menu bar in the CSS, and when I attempt to set the featured image it seems to overlap the right margin. It doesn't matter what size I make the image - it still has a shaded area that overlaps. I have had to add left margin, as the image seems to align with the previous size of the primary menu so blocking the left of the image.

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Regards, Paul