A new option has been added to the jpegtran tool and transcoding library:

-scale M/N Scale output image by fraction M/N, eg, 1/8

Currently supported scale factors are M/N with all M from 1 to 16, where N is the source DCT size, which is 8 for baseline JPEG. If the /N part is omitted, then M specifies the DCT scaled size to be applied on the given input. For baseline JPEG this is equivalent to M/8 scaling, since the source DCT size for baseline JPEG is 8. CAUTION: An implementation of the JPEG SmartScale extension is required for this feature. SmartScale enabled JPEG is not yet widely implemented, so many decoders will be unable to view a SmartScale extended JPEG file at all.