flashing pico with stm32loader.py

Is there a reason why espruino_1v87_espruino_1r3.bin is not working for pico

Yes, it's the image for the Original Espruino board (not the pico), and it uses a totally different processor.

Which image is needed for pico + shim with esp01 ?

Either the WIZnet or CC3000 works fine. I'd say use the WIZnet one though - the code is nicer :)