just dropping this here because it works, pretty sure though there is a cleaner way of doing things...
they are (see code just above).
totally agree, tbh, I just didnt get it to work...
The way the operation code works DepsgraphNodeBuilder::build_animation_images / BKE_image_user_id_eval_animation it works on related cow ID and I dont see how to set up a (working) relation to the Image directly, thats why I went
Time Source > Texture > Particle
Maybe @Brecht Van Lommel (brecht) can comment (but I will also check this again with fresh eyes next week...)