There are many concepts that evokes an image in the mind, a mental representation. As you said, this is the case for an apple. But try, for example, the christian notion of God. What do you "see", when you hear the word "God"? Of course someone could picture an old man in the skies, or a bright ligth, but we know that these are merely symbolic representation. An old man sitting on a cloud or a bright, ethereal ligth is in no way a TRUE representation of God. Think also about the root square of two. What do you see? The symbol "2" inside the "root" symbol? That is not what root square of two really is. The root square of two is a mathematical object, and only the definition is really able to convey the concept. I think it's the same for "Nothing". Even though, like in the case of God, I could still picture something while I hear this word (in fact sometimes I picture a black screen), I understand that these are merely symbolic representation, and that a black screen it's something, so it cannot be "nothing". But I still would separate the concept (the definition) and the mental representation (which may or may not be symbolic). Even though for some objects (like mathematical objects) I can't form mind images that are not symbolic, I know an objective definition, and that is the concept of those objects. Even if it is a negative definition, I still think that it's a legitimate concept.
Take the word "salumia". I just made it up. I will now give a definition of salumia: "Not a dog." Which should be understood as "If x is not a dog, than it is a salumia". You now have an operative concept of salumia. If now I say to you: "Is Lassie a salumia?". Your answer of course will be no. Lassie is a dog. Hence cannot be a salumia. Is Napoleon a salumia? Yes, Napoleone was a salumia. He wasn't a dog, so he is an element of all the salumia. I am a salumia too, and you too are. As you can see, it works witouth a problem. We have a word, we have a (negative) definition which provides an operative concept to distinguish a salumia from a "not a salumia" (which is now a definition of "dog"! Funny.)
And what do you picture when you now hear the word "salumia"? You may picture Napoleon, but you know that Napoleon is just A salumia, and only in a symbolic way could represent the concept that salumia convey. Only the definition is really true to the concept, because they are one and the same.
You could even define an apple with a negative concept. That would be quite difficult without specifying a finite set, so let's suppose we have a set of three elements: an apple, a dog and a cat. Let's call this set A.
Definition of apple: "for every x of A, if x is not a dog and x is not a cat, than x is an apple". Of course this is not a good definition of apple for everyday life, but I hope you get the idea. The idea is that there's nothing wrong in negative definitions. I think that Hegel had something interesting to say about these subject. And try maybe Berkeley and Hume for the "picture in the mind" part.
Finally, I think the definition you gave provide a legitimate concept of "Nothing". It is a negative concept, without a referent that you can picture in the mind, yes, but it's not a big deal. It can be conceived in the mind, but only as an operative negative definition. Not in other ways. Berkeley and Hume failed to understand this when talking about abstract objects, for example.
Finally, you say: if Nothing can be conceived by the mind, than it's something (something conceived by your mind), which is paradoxal. But to me to say that something can be conceived by the mind is to say that we have a concept of that thing. So we you ask me if nothing is conceivable, it's like if you are asking "Can we form a concept of Nothing?". The answer is yes. Does it mean that Nothing is Something? No. The concept of Nothing is something. Nothing is just nothing, as the concept say.