Okay, let's come at this from a different angle. A quick Googling reveals that we are not the first to ask if resizing counts as derivation. It has, not unsurprisingly, been the subject of discussion in several fora.

Flickr appears to hold that resizing is not derivation; this seems to be based on the following passage from the legal code for Attribution-NoDerivs 3.0 Unported:

The above rights may be exercised in all media and formats whether now known or hereafter devised. The above rights include the right to make such modifications as are technically necessary to exercise the rights in other media and formats, but otherwise you have no rights to make Adaptations.

(Emphasis by the poster who initially quoted this passage.)

Creative Commons itself says on the page for CC-BY-ND 4.0 (in a little definition pop-up) that "Merely changing the format never creates a derivative"; I am inclined to think that a resize is a change in format, especially if we do it as a thumbnail linking to the full image.

In an entry on her blog, Molly Kleinman, a specialist in "copyright, academic publishing, and scholarly communication in its various forms" (to quote her site), lists "Reproducing an unedited image on a website" as a permitted non-adaptation under ND, and adds

Creative Commons licenses do not affect your fair use rights. No Derivatives licenses do not prevent people from making fair uses of the work, which may include copying excerpts, creating parodies, and other activities that involve using the work without making an exact reproduction.

She also notes that you can simply ask the rights holder if you want to use a CC-licensed work in a way that is not permitted by the license; the ND license doesn't prevent them from granting additional licenses with more freedom.

So based on this quick bit of research, it appears resizing is permitted. We are not changing the content of the image, and we are exercising our fair use rights. If we really want to be sure, we can just email the Tate Collection and ask them.