Hi sunnysouthcoast and welcome to the forums,
I think that the best course of action would be to ask the publishers what they mean. However I appreciate that you may not wish to tip them off at this stage about your interest in the image(s) they control. I rather suspect that their terms may be influenced by the internet and other relatively ephemeral media, such as broadcasting, where repeat licensing is attractive amd lucrative. Obviously, for anyone producing a book or a journal, the only permission which can be used has to be for the duration of the copyright. I'm not sure what system of royalties might, even in theory, be applied to books or journals which could be in existence (or not) in, say, sixty year's time. I think the most sensible limitation on any licence, if there is to be one, would be the size of the print run.
I don't think you need worry about the extension aspect. If we are unfortunate enough to see future legislation which extends the copyright term, I think it is safe to assume that where a full duration licence is already in existence, there would be an implied or statute-based extension to the permission to cover the new extended period*. (cf
Regulation 21 of The Duration of Copyright and Rights in Performances Regulations 1995).
My other concern - which you didn't ask about - is on what terms the publisher is authorised to grant permissions. In some cases publishing contracts have temporal clauses in them through which the copyright can revert to the author or creator of the work after a set number of years. This should have the effect of limiting the publisher to issuing licences which only cover the duration of the term of the publisher's ownership. Clearly there is no point in having sunset clauses in a publishing contract if, once the copyright reverts to the first owner, there are no rights left which the new owner can exercise.
My last point is that you should look closely at the context in which the use of the images is being contemplated. I would imagine that in many cases the use may be justified by one of the
fair use exceptions, for instance for the purpose criticism or review of the image or the work it portrays, or less likely, for the purpose of quotation. There is no reason to suppose that 'quotation' should be strictly limited to quoting text or verbal utterances. It may be entirely pedrmissible to quote an infographic or even a table of figures, provided it can be justified under the fair dealing heading. So if the journal wanted to publish a review or critique of the work of an artist, photographer or sculptor, for example,
section 30(1) of the Copyright Designs and Patents Act 1988 might well permit an image of the work to be used without permission (although with a citation of the source of the work being used). There are no commercial limitations on the use of this exception. Even if you would prefer to adopt the safer (ie unchallengeable) route of getting permission it is worth using this exception as a lever to obtain favourable rates for any licence. Unfortunately the fair use ground which is probably the best fit for material which an academic journal would want to publish - for the
purpose of research - is not applicable, because that category of exception does not extend to commercial publication.
I hope this helps answer your question.
* Parliament and the English legal system generally dislikes laws which are retroactive. In the case of the 1995 copyright term extension, the decision was forced on us by the EU in order to bring all the member states inline with each other on this issue.