Who now remembers Hugh Gaitskell, once leader of the Labour party? He ignored a party-conference resolution and went on opposing unilateral nuclear disarmament anyway. This explains why Richard Hamilton was moved to make a Portrait of Hugh Gaitskell as a Famous Monster of Filmland (1964), now showing in a small, noteworthy exhibition at Inverleith House in Edinburgh’s Botanic Garden. Hamilton, you see, was a staunch CND supporter and therefore regarded Gaitskell (who was also a toff) as a nasty piece of work. So he sent him up.

You don’t need to know anything about Gaitskell or his beliefs to be grabbed by Hamilton’s “portrait”. This is because it relies not on its now-forgotten political purpose as on the skill with which Hamilton has combined contrasting images to make an attention-grabbing picture. Compare this with Shock and Awe (2007-8), a life-sized ink-jet print of Tony Blair as a nervy gunslinger in need of an orthodontist. It’s so clichéd and obvious, any trainee political cartoonist would reject the idea out of hand, together with the overworked title. Thanks to the many, scarcely noticeable, manipulations of several different images brought together here, however, the picture itself is nothing less than memorable.

All of Hamilton’s Protest Pictures are like this. The point is usually unexceptional, banal or naive, while almost all the images themselves are, in some way, remarkable.

Politics is also the point of some of the pictures in Foto, the Dean Gallery’s gripping show. It brings together about 150 photographs and photomontages created in central Europe between the end of the first world war and the beginning of the second, a time of social and political upheaval, of celebration as well as despair, qualities reflected directly or indirectly by the pictures. They come not only from Germany, Austria and Russia, but also from Czechoslovakia, Poland and Hungary, whose modernist photography is far less familiar here.

Though photography had been around for half a century by this time, the works here are so exciting and inventive, they look like the work of enthusiasts for whom everything was brand new. Photography was used in so many ways: as documentation, as art, and as scientific aid. It also provided the raw material for an entirely new art form, photomontage, which exploited the supposed objectivity of the camera in both propaganda and in the realisation of dreams — as in Marcel Breuer’s unforgettable Lonely Metropolitan (1932), where the eyes embedded in the palms of two levitating, disembodied hands stare disconcertingly out at you.

There is nothing political in the big show at the Royal Scottish Academy, however, unless you interpret the title, Impressionism & Scotland, as an assertion of nationalism. The unstated purpose of this money-spinner is, it seems to me, to give the public more of what it wants and thinks it understands — more impressionist paintings. Such a purpose obviously needs to be disguised by scholarly weeds, however. So we’re offered an account of the influence on Scots art of French painting in the later 19th century, as well as an examination of what Scottish collectors bought in that period.

I have nothing against premier-division masterpieces such as Degas’s L’Absinthe and The Rehearsal, or Toulouse-Lautrec’s At the Café La Mie. What’s more, I’m impressed by what Scottish collectors achieved at a time when the English were busying themselves with pre-Raphaelites — all the French pictures here once had Scottish owners. But while many of the French, genuinely impressionist paintings are thrilling, most of the Scots works are pretty dull — bar those by McTaggart and Lavery.

So, who were the Scottish impressionists? It turns out there were very few of them. To throw them together with the genuine article is confusing. Just one example of the maddening misleading: Van Gogh’s Moulin de Blûte-fin, a label tells us, was “the first post-impressionist to enter a Scottish collection”. But it’s a textbook example of him doing his best to paint like, well, an impressionist.

Richard Hamilton, Inverleith House, until Oct 12; Foto, Dean Gallery, until Aug 31; Impressionism & Scotland, RSA, until Oct 12