Use the Base64DecodeToFile that you can find in the community commons. (and a entity image that you created an have sufficient rights, not directly the system entity)
Also: don’t forget to give your file a approriate name + extension
E.g: this is to decode a pdf file so the doc entity has a generalisation of system.document file. You should use the system.image but ths flow should be the same. This download the file, you could commit it.
Did you succeed?