Remember that you should not store photos on iCloud. It's time and capacity limited. It holds up to 1,000 photos for no more than 30 days. So, import no. 1001 and the first one is dumped. Leave them untouched and they will be deleted from iCloud after 30 days.
You can't delete individual photos from iCloud (only reset the whole thing) so trashing from a device will not trash from iCloud.