I have to agree with Dark SpilveR here. The assets "page" is not a page per se. It's very common for a site to have an assets folder, or for high traffic sites, a whole server dedicated to assets. In web programming, a page is made up of a number of things, such as the html of the page, the text content, scripts, stylesheets, images, videos, etc. "Assets" are the individual pieces of these pages and typically include scripts, stylesheets, images and various other media (e.g. videos). Thus, the image is there on the assets server, just sitting there, waiting to be used on some page somewhere. No credit is necessary for an image at rest.
Also, in looking at the source of the page in the article the image is used in, one can see that the image has different sizes, presumably for different platforms (desktop, tablet, mobile). The one you referenced is the smallest image. There are two other sizes referenced, one medium, and one large. There appears to be some script that they use to place an appropriate sized image on the page for layout purposes, likely depending on the device (desktop, tablet or mobile). Both the medium and the large image have the credit embedded directly in the image. It's completely possible that if the smallest one is used, the credit is made elsewhere outside of the image in the page it's composed on, or it's possible that the smallest image is never used on any page even thought it's in the "small size" script variable.
http://assets.jw.org/assets/m/g10/201011/g10_201011.art/102010407_univ_cnt_3_md.jpg
http://assets.jw.org/assets/m/g10/201011/g10_201011.art/102010407_univ_cnt_3_lg.jpg
I believe, in this case, there isn't an issue.