In your case, siphs.png would be the filename for the “danielsale’s link blog” posts. And, daniel-sale.png would be the filename for the “Daniel Sale’s Blog” posts.
The way you can tell what icon name to use is — go to you lifestream page and view the source code (Ctrl+U will show it in FireFox or Opera). Then scroll to the lifestream list and make note of the class defined in the lines that start with: <li class="item"><a><class ="classname" href=...>
where classname = lastfm, digg, siphs, etc.
I’ll try to think up a feature that makes this information more easily available, but that’s how it’s done for now.
Another thing I noticed is that the icon for BlinkList is not correct in the lifestream. The BlinkList icon is present on Profilactic, but in the lifestream all I see is an RSS feed icon and the name of the bookmarked site.