I rarely delete a photo from an online host, so it's unlikely to be a problem for my images (and if there is an issue, let me know because it's a problem with my whole account.)
If people are linking from free accounts, they need to check that the links still deliver if they exceed maximum uploads. Some sites like Flickr will let you show the latest 200 posts, but will still host photos that are in excess of the 200 (it's a kind of underhanded prompt to get people to pay annually for the full account.)
Other free accounts will purge photos that exceed your allowed settings.
If you are using Flickr, and set an album to private, Flickr will no longer show your photos where you've linked them, the photo has to stay public.