Wrong image being displayed…

I have an issue with one of my sites. It’s one that I didn’t create to start with but have heavily edited. I’m in the process of rebuilding it. It has a front page feature image of 960px wide. The main body image are 640px. Normally an image intended to be used as a feature is uploaded as 960px, but an image intended to be in the body of the post is uploaded as 640px.

Now if we decide to add a feature image to a post and select the size (640px) from the size menu it adds the code, as expected, indicating the 640px version of the file. But for some reason, that I can’t work out, the browser shows the 960px version instead.

1. If I take the address from the code and paste it into an empty browser window it loads the correct file.

2. But if I right click on the image after the page has been rendered by the browser and select to open it in a new tab it loads the full size image, the original (contains no image size added to the name).

Any thoughts as to why?

Thanks, Cheers,.