User blog comment:Ḡwẵine Ḹٍٍkƨ Ĺiĸe Ͼềлȑềd/Wallpaper/@comment-180964-20120425000607/@comment-180964-20120426202023

Not sure off the top of my head but I'll look into it. If I had to take a guess right now I think it was around time just before Series 4 officially started an I think they also came out around the same time as the Episode Press Release as well.

Update: It seems the main cast promo images were released on September 21, 2011 and The Darkest Hour Part 1 first aired on October 1, 2011. So that seems to be the time frame as to when the Series 5 promo images will be released. However it also depends as to when Series 5 Episode 1 will first air as well.