There is a difference. Your photograph was still within it's copyright period.
Copyright extends to 70 years past the author's death (in the UK). Since you are still alive, any photograph you take is obviously still within it's copyright - with one caveat.
The caveat is exactly what is at issue. You cannot claim copyright on a direct copy of someone else's work. You CAN claim copyright on a DERIVATIVE work. So the question here is was the photograph of the painting a derivative work?
The goal of the photograph was to reproduce the painting in digital form exactly as it was meant to be viewed in the gallery. No changes were made, and the painting was not used as the basis for a new work (even a person standing next to it within the shot is a 'derivative' work). Which would mean the photographs in question were NOT derivative works, but merely digital copies of the original, and not copyrightable in and of themselves (they would piggy back on the original copyright).
Which means any digital image of a painting that is in the public domain, is also public domain - as it is not a derivative artwork. And you cannot steal something that is public domain.
In your case, yes, someone stole your art work, because it IS within it's copyright. And regardless of whether they stole a print, the digital image, or any other faithful reproduction of your photograph, it would still be counted as illegally copying of your work. But 70 years after you die, anyone and their dog can take your image with impunity. They can even take the image of someone else's photograph of your image on a screen (as long as it's not altered in any way).
So as someone else said, yes, it might take a lot of technical expertise to faithfully capture these images to do them justice, but that does not suddenly give the person taking them copyright on what is, essentially, a faithful reproduction of the original artwork not a derivative.
By way of example, how about we take the works of Shakespear. Originally released in manuscript form. Someone took that manuscript and typed it into computer text files. Does that mean that person now has copyright on the resulting text file? It certainly took a lot of effort to transcribe the text - but it doesn't matter. Just because it took a lot of effort to do something (it takes a lot of effort to paint a forgery, too) does not infer copyright - if the text is identical to the original shakespear, then it is just a faithful reproduction on another medium and still int he public domain.