Hahaha imi place ideea!
To all ppl here who don't get it: 'You see green horses on walls' is kinda like 'What on earth are you dreaming about?...green horses on walls or what?' -basically saying that you've got your head in the clouds. So this is illustrating a strange saying
(In English you could take 'It's raining cats and dogs' and draw a picture of a rainy day with cats and dogs falling out of the sky XD )