BTW, I looked at the source and it appears that the text "thumbnail" comes from the alt text, which, according to http://www.w3schools.com/tags/att_img_alt.asp, should provide "alternative information for an image if a user for some reason cannot view it (because of slow connection, an error in the src attribute, or if the user uses a screen reader)."
I'm certainly not an expert, but what WordPress does in my case seems wrong to me for two reasons:
- "Alternative" to me means that it should replace the image, not display on top of it.
- Even if it did replace the image, the text "thumbnail" makes no sense in any of the applications w3schools gives. I wouldn't want a screen reader to read "thumbnail" in front of the excerpt.
Just for reference: I also found the following, which may be pertinent to this issue: