Hi S1mons,
The problem with this as a business plan, is that you need to be 100% sure that the works you obtain from wikimedia or the Yorck Project or any other source are confirmed as being in the public domain. This may require a lot of research on your part.
The consequence of failing to do this, and accepting someone else's assurance that a work is in the public domain, could be financially painful.
There are two aspects to what you need to check: that the artist of the original painting died more than 70 years ago* (relatively easy with known artists), and that who ever photographed the painting is not claiming copyright in his photograph.
This second test is far more difficult because different jurisdictions hold slightly different views on the position of such photographs. In the USA the frequently quoted case is called
Bridgeman Art Library v Corel Corp, which held that since copying a painting by photographing it adds nothing original, the photograph itself cannot attract copyright which is reserved for original works. However in the UK and Europe the position is somewhat different and a photographer who can show that he/she used skill and judgement in the choice of lighting, colour correction and alignment along with several other factors, may well be entitled to copyright in his/her photograph because these unseen qualities amount originality, or as the Court of Justice of the European Union has found, the work reflects the personality of the photographer. The problem is that there haven't been any recent cases to test this divergence of views.
Wikimedia, being an American company, relies heavily on the Bridgeman case as grounds for claiming most of its artworks are in the public domain. I haven't discovered the Yorck Project's detailed explanation for how it confidently says its catalogue contains PD items. That said I am not aware of anyone having brought a case against Yorck, similar to the one brought by Bridgeman, so maybe they are fully justified in making their claim.
Once you are confident that the work you wish to use is in the public domain, then you making changes in the way you suggest should not cause any copyright issues. However you do need to be aware of the moral rights that artists have for their work not to be treated in a derogatory way (the so-called
droit d'intégrité). Once again the effect of this varies by jurisdiction. The US has very
weak protection for moral rights; in the UK the
moral rights only last as long as the copyright, but in France moral rights exist in perpetuity; Germany also has a strong tradition of protecting the moral rights of authors and artists, but their legal code does include one helpful limitation on authors:
"Article 39 Alteration of Work
(1) The holder of an exploitation right may not alter the work, its title or the designation of author (Article 10(1)), unless otherwise agreed.
(2) Alterations to the work and its title which the author cannot reasonably refuse shall be permissible.
So unless the artist was French, you probably won't have much problem with the subject, and you are not too extreme in the changes you make, which might affect the posthumous honour of the artist!
* If the 'Wahol' you mention in your posting is Andy Warhol, then of course he has only been dead for a mere 27 years so his work is still in copyright.