Explicitly supplying some other value for the size option should already override whatever has been set using plots:-setoptions. That's a nice thing about plots:-setoptions, it allows you to set preferences, which can then be easily overridden at any time, with the usual command calls.
Also, the size=list option can be utilized on a call-by-call basis, with different values (as the first float in the list, designating the proportion) as desired.
When you write of "images" I'm not sure whether you mean images as backgrounds to plots, or inserted from the main menubar, or embedded using ImageTools:-Embed, or as a Drawing (canvas).
For images used as the background for 2-D plots, those don't respect plots:-setoptions with the size option, which I think is a good thing. But an explicit use of the size option in a plot command can still override the original dimensions of the image. Eg, using Maple 2016.2, with each of these lines in its own execution group, say,
img := cat(kernelopts('datadir'),"/undercarriage.jpg"):
# Good. Image file is 600x400.
# Should not adhere to setoptions preference of 300x300.
# Good. Respects the specified size.
# Good. Picks up setoptions preference of 300x300.
For images inserted using the main menubar's Insert->Image action, you could insert them into a single-row, single-column GUI Table, without exterior borders. By right-clicking on that Table you can set its properties. That includes setting it to be a fixed properotion of the GUI window's width (default is 100%). You can also center that borderless Table. As long as the original image width is greater than that proportion of the window width then the image will be zoomed to that dimension. So just make sure that the image is a wide as the maximally needed case.
For an image embedded with the ImageTools:-Embed command you can also right-click and change the Table's properties to a specific fixed proportion of the window width. You can also center it, eg, using the main menubar's Table->Alignment->Table->Center . (It would be nice if that command got the nicer options to do all that programatically like DocumentTools:-Tabulate can, or alternatively if DocumentTools:-Tabulate got the functionality to handle Array-based "images".)
Unfortunately the Drawing canvas does not seem to rescale automatically when placed within a GUI Table whose width property is set to a fixed proportion of the GUI window's width. Ie, objects drawn don't rescale when the GUI window is resized or maximized. That's a pity.