Sorry, I’ll give it another try. English is not my mother tongue… 🙂
The “Home” page (http://demo.onedesigns.com/pinboard/) shows the attached article image with a width of 322px and is determined by the width of the column. Therefore the image can not be wider than the “Home” page column.
The “Astronomical Clock” page (http://demo.onedesigns.com/pinboard/?p=212) has the same image attached to the post, but this time not as an “article image” but as a “standard image”. The image width is determinded by the cloumn size of the page, which is more or less (depending on the space between the column of the “Home” page”) double the size of the “Home” page column width, as it spans over two columns.
The final picture when clicking on the image on the “Astronomical Clock” page is defined by the size you selected while uploading the image.