[Norfolk Newby]

The tractor has just been washed in the ford. The old man is complaining about the water being polluted by the nasty tractor (oil, grease etc.) ans scaring the fish.

Around the time tractors started to appear, it would have been normal to take the farm horses to the ford to give them a wash, so why not take the tractor?

[SallyintNorth]

The picture is at least an nonsensical as Dizzycow's suggestion!

So we think it must be a whimsical cartoon, possibly with a caption such as, "You can lead a tractor to water..."

[Dougal]

I suspect that the idea was that since at some point the over dinner chat would be rubbish they decided to give you some thing to chat about. It's not a rat either, dimensions are wrong, more a rolled up ferret me thinks!