Longdesc tag

Hello there
I would like to ask you for advice as for using the longdesc tag in html img tag.It turns out that this tool http://wave.webaim.org is saying that the longdesc attributes for the img tags of my site do not contain an url. I do not understand that because they do conatin urls. However, after that I have seen some places like http://en.wikipedia.org/wiki/Longdesc_attribute where it is not very clear whether it is good to use it or not. So, now I am wondering how to fix the problem in case the tool http://wave.webaim.org is right or otherwise whether I should remove this longdesc parameters from the img tags.
Thanks in advance

It might be worth reading this page from the SitePoint reference:

The longdesc attribute is not well supported, unfortunately.

The odds are you do not need to use them for the images in question and the TITLE attribute will suffice. Very rarely have I come across an image that would really benefit a lot from LONGDESC attribute.

In addition you have to consider whether you have supportive text within close proximity of the image, for example; within a descriptive paragraph near to the image.