Looking at the images they look pretty nice as presented, however something odd is going on with the scaling. When I view the image on my monitor the last image is 256mm wide. If I click on it, it displays bigger and is 292mm wide. To compare I went into Photoshop as I know it displays pixel for pixel and a 1500 pixel image there is 350mm wide, while a 1200 pixel image is 292 mm wide. It would seem to me that the recommended pixel dimension is 1200 px to fit the column width at least as it scales on my display in Firefox.
I can't notice too much difference between the image in line and the larger one when I click on it, but the image I suspect is not one that will suffer too much from compression, other images may fear differently. I'm viewing on a 27"display 2560 x 1440 pixels.
As far as quality goes the biggest impact will come re-sizing the images - I don't notice much difference between a 50 and 70 quality image in most cases.
Ideally it should display in line at native pixel dimensions for best quality - 1500 pixels is clearly too wide to the column here but it does display nicely if you click on the image.