I tried switching Interpolate to ‘nearest’ for a new experiment I’m working on, and this does indeed solve the problem. (It might be that the fundamental issue is solved in the most recent PsychoPy version too, I’ve been asked to keep to an older one).
My graphics card, in case it’s relevant, is: Intel Iris Graphics 6100 1536 M
Thanks!
Update in 2021:
I hadn’t found it when I made this thread, but there is another thread describing the same issue that has some more information: Borders on some images - #6 by ewing
Turning linear interpolation off/using ‘nearest’ interpolation worked for the particular experiment I mentioned in this thread, but often times this causes issues with image quality. Because of this I’ve unmarked jon’s reply, to make it clear for anyone stumbling upon this thread that the issue is on-going (unless it’s been solved in the very latest PP version).