The Journal is copyright of the the BAA. Individual articles, illustrations, etc. remain the copyright of the author or the imager, whose permission must also be obtained.
I suggest you contact the author of the article in which the image you are interested in appears. In my experience this is usually all that is required. They may request you give some sort of credit for using the image and you might like to consider citing the BAA Journal. It’s worth bearing in mind they they might not own the copyright either, but have permission from the actual copyright owner to use it for the sole purpose of illustrating their article – in which case you need permission from the actual owner.
I hope this helps.