First, angel is a noun, not an adjective. Angelic is the adjective. Second, it is possible to think of things that one has not observed, for example, a person in a rage envisioning harming an enemy, or a person under the influence of drugs having hallucinations, or a person undergoing considerable stress or strain envisioning things that did not ever enter their field of vision, or observation, prior to having the imagined visions. Further, it is a matter of course for novelists to create entire communities that they never observed, but that were products of their imagination. Regards, John Companiotte, Atlanta, GA USA

We "thought" up elves, dragons, ninja turtles, and so on.  Just because we can "think up" - imagine - other forms does not give them existence.  They "exist" only in name. My worry is that most people don't realize that sounds-given-meaning inform the mind.. When they hear "angels" often they believe these creatures actually exist.  This does not help people solve problems individually or collectively.  
The idea of "God" was thought up by the Sumer in 9000BCE. They gave the penis names as supreme creature,  Since then the masses believe "god" exists as the moral guide of all.  This has done such great damage to our species, like climate warming, it may no longer be repairable. 
Louise Goueffic   

What if I have only heard or read about something that I have no reason to think does not exist, but I have not observed? Could I not form an image of it based on a description by using likenesses and extrapolations so that my image is not really an "assembly of selected parts of what has already been observed" but still would require some kind of prior observations as a basis?

And this implies that one cannot think about God except those who have already observed God (since God inherently has no parts).

Which is another reason why the original formulation is faulty.

Faulty or with a high-degree of a naïve-like understanding of Late Wittgenstein.
I think that the original formulation tried to show us a language-game reasoning, but without any clarity.

There is no 'something' you can think of which isn't a collection of experienced stuff. Thoughts give rise to a new combination of experienced information in memory, there is nothing in memory which isn't experienced or isn't a collection of experienced pieces of information.

In language of philosophy investigations,  It is not an issue of proving existence but rather the fact that we can converse at all means we are in agreement as to what is meant when we say angels.
It is not within the scope of logic or language to prove the existence of objects/concepts.

Well, that depends on how you want to slice your bread. Maybe we couldn't imagine a circle if we had only seen straight lines, who knows? Certainly anything I think of has some kind of resemblance to something I have experienced before, but if we define this that broadly, it becomes pretty meaningless.

Thinking is not the same  as imagining.

I could think about something, if i don't see it, buy I have heard about it to think about it, in other case I could imagine it or use fantasion. Sometimes i see something but I do not can name it.

Terry, 
I agree that proof is not the point, especially with regard to that which is "incapable of being seen or perceived in a manner that one can explain" i.e., linguistically validated" (scientists and mathematicians know the limitations of even numerical language).  I believe that the most important "glue" of our existence falls into this category, such as, love.

It is misguided to pursue proof of the divine mysteries.  We are seeking enlightenment and collective wisdom, the water and sunshine of our growth.  We need nurturing reciprocal inspiration through our passionate dialogue, I believe.

All the best, Maria

Dear Tami,

Devinely put, madame... gracias

And thank you. :-). 

Good question.
Guess one should not talk abou imagining without some kind of observation of something 'outside' your mind. Can you think (in words) without imagining them?