It is all too common seeing missing, externally linked photos on various web fora, which can completely ruin the impression of an exchange. Even the large image hosting sites go missing at regular intervals, much more so that small, privately run community sites. Alternatively they suddenly change they policies regarding external linking, once their investors realize they may have a broken business model.
As a consequence I always make a point of scaling and uploading photos to the main site whenever possible.
People will 'borrow' nice photos no matter where they are hosted, so you are not relinquishing any control by uploading them to a community site. You don't have any real control to start with. Think Google images, so either choose to upload your images to the 'net, or don't. There are no degrees of freedom here.