What tools support BPG? E.g. can you use imagemagick to scale it / convert it to JPEG? (And if so, from which version?) Does GD handle it? VIPS?

What's the browser support for client-side decoding? Does that work on mobile devices? What's the overhead?

On a first glance, this might be more interesting for mobile, as highly lossy compression is where BPG beats JPEG the most.

*Mozilla did a study of various lossy compressed image formats. HEVC (hence BPG) was a clear winner by a wide margin http://people.mozilla.org/~josh/lossy_compressed_image_study_july_2014/. BPG files are actually a little smaller than raw HEVC files because the BPG header is smaller than the corresponding HEVC header.
*BPG natively supports 8 to 14 bits per channel when most other formats use 8 bits (including most of the JPEG implementations and WEBP). It gives a higher dynamic range (which is important for cameras and new displays) and a slightly better compression ratio (because there are less rounding errors in the decoder).
*BPG uses high quality decimation (10 tap Lanczos filter) and interpolation (7 tap Lanczos filter) to handle the chroma samples in 4:2:2 and 4:2:0 formats.
*BPG can be supported in hardware with standard HEVC decoders and encoders (it uses a subset of the Main 4:4:4 16 Still Picture Profile, Level 8.5).