King Charles' Fiery Red Portrait Vandalized with 'Wallace and Gromit' Pic

King Charles' portrait got a bit of a facelift -- an animal rights group threw some artwork over it in a museum as an act of protest ... and now, Chuck looks a bit stop-motion-y.

HRH's fiery painting -- which is currently on display in London at the Philip Mould Gallery -- was vandalized Tuesday by a couple of demonstrators who stormed in and did a quick hit job on the artwork ... and while it wasn't all that destructive, it did leave an impression.

Check out the video posted by Animal Rising -- the group taking credit for this criminal act ... two guys walk up, plaster a 'Wallace and Gromit' face cutout, plus a quote bubble.

The quote reads, "No cheese, Gromit. Look at all this cruelty on RSPCA farms!"

As for what the protesters are griping about exactly -- it has to do with the RSPCA org ... which Charles recently became a Royal patron of ... and which works on improving the lives of farm animals. Animal Rising, however, clearly believes RSPCA does more harm than good.

We've seen similar protests like this before -- especially at museums, when animal rights activists throw soup or paint on artwork. Now, though, they're taking aim at the King!