First of all thanks to all who always help me in the forums with my questions :) now I try to give back a bit of infos:
Here I give U a short tutorial of how to convert all images at once in 8 bit images.
File -> Batch Process -> select all files to convert in the FILE of TYPE slot ! and click ADD
click on NEXT
BATCH OPTIONS choose EXPORT...ADD -> Settings: CUSTOM click EDIT
in the new window select Format : PNG 8 and ALPHA TRANSPARENCY ! click OK
click NEXT ! select a location for BATCH OUTPUT and click BATCH
and here we gooooo :) perfect alphas for aaaallll ur sprites at 8 bit :)