Hi @Fabien_Couprie,
indeed, the task is easily formulated but quite hard to implement, with KNIME at least 
This is what I propose: stitch_images_example â KNIME Hub
I hid the magic it in a component, here is how it works: It uses the âGroup Byâ node with the compose image aggregation at the heart of it, when the offset metadata of the images is set previously. Youâd need to give the X- and Y- dimensionality in the component (this could certainly be done automatically, i.e. with the âImage Propertiesâ node if need be, but I wanted to keep it simpler). For each Y and X, dimensionality is set as the images offset in its metadata and the actual composition is performed with the group by node. Inspired by @gab1oneâs post: How to join box plot images into 1 image - #4 by gab1one
Let me know if that works for you or if you need more explanation!
Best Regards,
Lukas