Well if you follow the link, it describes the different methods of scaling much better than I can here.
Most notably are the first three types. The first two are equivalent to what you have in Construct for the sake of speed. Then the third is the most commonly used, when speed is not an issue.
So without being trying to be completely inappropriate, its just not an image viewer, and wasn't designed for large "high-res" images.
[quote:3czly5oh]Rule of thumb
Scale down less quality loss.
Scale up high quality loss.
Is actually a big hint on how to deal with the issue.
Another would be that you can shrink images in the layout editor without quality loss.
And that when you take them back to their original size(runtime) there is no loss either.