I don't think the modified ones need to be content-addressable, just the originals. Imgproxy does this on-demand for images, so I don't see why you couldn't just use that or a service like that as a front-end for blossom. This would allow you to trade-off referential transparency for performance depending on reader use case. Of course, there are also many files that publishers don't care about censorship resistance and verifiability for, but in that case I would just use a traditional image host. 

 And if you want referential transparency for a transcoded one, then transcode it and then upload it to blossom