Leeching bandwidth? Really? This is not 1999 anymore, no one cares if their images are hot linked anymore. Be ready to be surprised, people found ways of making money from hot linked images.
Be ready to be surprised, the next link to a meme you click could be a gore-fest.
Some of us are still on shared servers or have limited bandwidth, or just care on principle.
When the leechers are popular forums it can be a definite tax.
Making a local thumbnail isn't that difficult, likely in whatever language you happen to use.