How do I add a large sized image to my post?

After you’ve uploaded your image, or when you’ve found one in the Media Library tab of the uploader box, you’ll see some options for image sizing that look like this:

Choose the Full size option near the bottom right, then click Insert Into Post at bottom left.

The image will be added to your editor page.  If the image is very large it will be automatically sized to fit the width of your theme.  You can resize it to any size you like:

Click on the image in the editor window to highlight it.  You’ll see some small boxes appear on the sides and corners of the image; these are resizing anchors.

Click and drag the anchor at the bottom right of the image to resize it larger or smaller.  Provided you use the corner anchors only the image won’t become stretched or distorted.

Be careful when making an image larger: it will become pixellated if you make it larger than the original, and some themes will have sidebar problems when a post contains an image that is larger than its content column.