https://www.wowinterface.com/downloa...Converter.html
Did work last weeck, I used it to modify objecticonsatlas.blp but I needed to update CascView because it extracted files which where not usable.
I run Blpng as Admin and it works fine for me, if you get an error about a missing dll you are most likely missing a Visual C++ Update or something in your system is broken, I have read about this some time ago, just google the dll or look on youtube for solution, there are some valid fixes.
Edit: Forgot to mention that most online converter I tested did fail with some newer blp files from Wow, which seem to have some major difference I don't know of. I get the same broken picture file CascView produced before i got it up to date. Some files might convert online just fine, objecticonsatlas.blp did not when I tried online solutions.