How is the image quality of the Canon EF 50mm f/1.8 II compared to the EF-S 55-250mm f/4.0-5.6?

I have both lenses. When the 55-250 lens has been used at the 55mm end I can generally (but not always!) tell the difference between that and one taken with my 50mm.

The 50mm is generally sharper and higher contrast. The depth of field of the f/1.8 max aperture is also a giveaway.

Prime lenses tend to be better in all aspects than zooms, and here the difference is not that major. The only actual advantage is the much larger aperture, while quality is slightly better. The 50mm is also a little bit louder in auto focusing, and may hunt a bit more in low light before it gets spot on auto focus.

Here's a Canon Lens Guide - http://www.the-dslr-photographer.com/which-canon-l...

That's impossible to tell without direct comparison... that is, taking a photo of exactly the same scene at the same time of day from the same location. I don't believe you're going to find anyone who's done such a comparison.

I can tell you, because I own one, that the 50mm f/1.8 is very sharp, especially for portrait work in natural indoor light as from a window.

Normally speaking a single focal length lens out shines a zoom lens all the time. That's why Pentax (my brand camera) offers "limited" lenses for that extra quality, and they are "priced" accordingly! Even the standard single focal length lenses are better. But you will never see it if you are a 4x6 only print maker!

The same. It's how you set them up. Actually, it is expected to get more blur at f/1.8 compared to f/4.0 given that everything else is set equally. It's how you use it. The lens is just a tool.

prime lenses are always sharper than zoom lenses.

I'm not sure how you came to compare two totally different things like this. It's like asking us if a bicycle or a car is better. Each has its place and purpose.