Thanks for sharing that - I can see what the problem is now.
You have this code:
set you to ( recognise image ( costume image ) )
The costume image block returns the contents of the image
The recognise image block returns the results from using your machine learning model to classify something
Combining these into
recognise image ( costume image )
returns the result from classifying the costume image
In other words, this will return the string "Stein", "Papier", or "Schere".
You aren't adding image data to your training buckets. You aren't adding pictures, you are adding words like "Stein" into the training buckets.
The error about the unsupported image format is referring to the fact that the words you are adding to the training data aren't images.
Kind regards
D