swfextract from the swftools package should be able to do it. `swfextract file.swf` should list resources in it, then `swfextract -p 12 file.swf` will get the

swfextract from the swftools package should be able to do it. `swfextract file.swf` should list resources in it, then `swfextract -p 12 file.swf` will get the PNG with id 12.