41 questions are tagged with both wpf and imagesource : http://stackoverflow.com/questions/tagged/wpf+imagesource
And as kiamlaluno noted, 24 questions are tagged with both c# and imagesource : http://stackoverflow.com/questions/tagged/c%23+imagesource
The union of these seems to represent the set of imagesource tagged questions referring to the
ImageSource class within the .NET framework. They don't intersect on all questions, so the total number of relevant questions is a little under 50 (a quick count was 7 c# imagesource questions which looked relevant to
ImageSource that were not tagged with wpf as well).
The rest did look like a mix of questions either related to the
src attribute of the html element
img or some other concept semantically related to the sourcing of an image in some form, sometimes with no other direct reference to "source" other than the use of the tag. I think I saw only one imagesource tagged question which referred to an class or class member named ImageSource which wasn't tagged C# or wpf, but it ended up being about winphone7 development, and so still referring to the .NET class of
I would agree that either the tag should be renamed to
.net-imagesource or it should at least be definitionally specified as referring to the .NET
ImageSource class, and questions using the tag in other ways should have it pruned. Because the tag is currently used somewhat ambiguously, I'm not comfortable editing it myself to define it as being for the .NET usage specifically, but I do feel that would be the most useful solution.