Pictures at different resolutions

I built wp-dev.cpime.org on a laptop with a screen resolution of 1366×768. Pictures were sized to extend from the left margin to the right yellow sidebar. However, when viewed on a HD screen the pictures do not extend to the yellow sidebar as you can see here: http://wp-dev.cpime.org/how-we-help. Of course, if the pictures are sized for HD then they extend into the yellow sidebar.