source:*twimg* jpeg_artifacts
source:*twitter* jpeg_artifacts
And even more still that have not been tagged with jpeg_artifacts. The ammount of artifacted images we've been getting lately has increased drastically and more and more are being uploaded daily.
A great deal of artists will eventually upload higher quality versions to pixiv/seiga/nijie/blog/etc, so uploading from their twitters should be avoided. Even PNGs from these artists should be avoided since twitter tries to resample them to reduce their size, just like they do to JPEGs, but instead of actually being intelligent about it and converting those PNGs into JPEGs, they keep them as PNGs and only end up increasing the filesize. It's more or less the same problem we had with pixiv and their scrubbing of meta data from images (topic #9745, thankfully they seem to have stopped doing this).
I'm not calling for an outright ban on uploading from twitter, since unfortunately there are some artists that post exclusively to it. However, a bit more care should be taken when uploading. If an artist posts their art only to twitter and said art isn't horribly compressed or is a PNG, then uploading should be fine. However, if the artist routinely uploads their twitter posts to other sites and at a higher quality, then avoid uploading from their twitter.