NS blocks email images, but after I D/L manually it deletes them

I'm on Win 7 x64 with latest NS installed and using Office 2016. I have NS set to block images on emails so I can choose which ones I want to see, but any I D/L disappear sometime in the near future. I then have to D/L again which is frustrating as the images may no longer be available to D/L. I would have thought manually D/L images would in effect be me saying they are safe. Why then does NS delete them without warning? I have searched the forums and found nothing about this yet, is there an option somewhere to allow what I want?