Based on your last paragraph, you might be interested in Thomas Nagel's The View From Nowhere. In that, he argues that it is impossible to achieve a completely objective perspective--- what he calls the View From Nowhere. This isn't directly related to your first paragraph, but something you might enjoy.
As to your first paragraph, you might find this book interesting. Locke had some interesting ideas about the limits of imagination. For him, what is imagined is always some manipulation of things actually experienced. So, for example, you can only imagine a Centaur (half-man, half-horse) because you have experience of both a man and a horse--- or at least things relevantly similar.
A similar sentiment is echoed in Descartes's First Meditation. Check out section 6 and the discussion of painters.
As to imagining "Nothing", I'm inclined, along with you, to think that this is impossible. It seems to be no different than thinking about nothing. But then it seems like there is something you are thinking about, namely, nothing!
UPDATE: It occurred to me that given the Ontology tag in your question, and given that my last paragraph is mostly based on my own idiosyncratic views about existence and reference, I should bring in some considerations from the seminal article on ontology, Quine's "On What There Is". Your questions about nothing, and my own reasons for thinking that imagining nothing is impossible, bear a striking resemblance to the problem of negative existentials. Some philosophers, notably the Meinongians, have thought that there are some things that have the property of "not existing". So, they would analyze negative existentials like "There are no unicorns" as expressing the sentence "There is something such that it is a unicorn and it doesn't exist". They could do this because they distinguished between two senses of "there is". One, the one familiar to us from Quine, is to read "there is" as expressing the existential quantifier. Anything that "there is", in this sense, exists. Now, the other sense of "there is" is subsistence. They thought that there are some things (like unicorns, for example) that subsist but do not exist.
Quine thought that this talk of there being things that don't exist was a bunch of nonsense. He held that "there is" only expresses the existential quantifier and that anything there is must exist (as an aside, he famously, but uninformatively, answers the question "What is there?" with "Everything"). But then how did he analyze our earlier sentence about unicorns? He would analyze is thusly: "It is not the case that there exists something such that it is a unicorn" (sorry for the quasi-logic speak, I really want to regiment this in first-order logic a la Quine, but can't seem to get MathJax to work on this SE). For Quine, this sentence carries no presupposition of anything's existence, much less of a unicorn which subsists but does not exist.
Bringing this back to the original question about "Nothing". If I put on my Quine Hat, I might say that to imagine nothing is simply for it to not be the case that you are imagining something. But that isn't very helpful, is it? Well, let's suppose (as we seem to be supposing in this example) that imagination is object oriented, so that whenever we imagine, there is some object of our imagination. What this pseudo-Quinean view would hold, then, is that to imagine nothing is simply to not be imagining any particular thing or collection of things. So, for example, a dead person is imagining nothing. I imagine (chuckle) that this view would deny any "objecthood" to "Nothing".