Use minha ferramenta para remover o canal alfa deles: link
Remove o canal alfa dos arquivos BPG antigos, tornando-os decodificáveis pelos decodificadores BPG mais recentes.
$ bpgdec -i test.bpg
size=1606x1263 color_space=YCbCr alpha=1 premul=0 format=4:2:0 limited_range=0 bit_depth=8 animation=0
$ bpgdec test.bpg
Could not decode image
$ bpg93_remove_alpha.pl < test.bpg > test.noa.bpg
magic: 425047FB
PixelFormat: 1 alpha: 1 bitdepth: 8
ColorSpace: 0 ExtPresent: 0 Alpha2: 0 LimitedRange: 0 ReservedZero: 0
Width: 1606 Height: 1263
Picture data length: 81969
Alpha data length: 239
$ bpgdec -i test.noa.bpg
size=1606x1263 color_space=YCbCr format=4:2:0 limited_range=0 bit_depth=8 animation=0
$ bpgdec test.noa.bpg
$