valheru:
2. Access the MicroSD by i2C from the other board, requesting an image, buffering it and then getting it out -> it would toggle the master/slave mode in both Arduino and NodeMCU to have a bi-directional conversation everytime. I guess it !
That'd be the way, but why would you switch the master/slave roles?
The NodeMCU will be the master, as it has to request the data from the Mega - be it sensor data or the image. The Mega can just stream (no need to locally buffer beyond the I2C buffers) the whole thing to the NodeMCU in one go.