If you're not my client waiting to get my high-res file, the answer is no. No. No. No. While I really don't have to give you a reason for this, I'm going to anyway. Because I'm cool like that.
Because I'm Not Allowed To
The first potential reason that I may not be willing to give you a high-res version of a piece is that I legally don't have the right to. You see, many companies I work for buy up all usage rights of the images they commission save those allowing for me to use said images for self-promotion or to make prints. What this means is that I can use the image in my portfolio (printed or digital), on the various websites I've uploaded my work to, in mailings to art directors, and so forth. Basically, if I'm using the image to fish for work, I am using it for self-promotion.
Giving you a high-res version of the piece does not fall into this category, and so I am obliged to decline. You see, spreading around high-res versions of work when I don't have the right to endangers my livelihood. I'm not interested in living on the street, so I play by the rules of my contracts. If you still want a high-res file, I will direct you to the company that owns the image and you can take it up with them directly. Good luck with that.
Because It Might Hurt Me
The second reason I may not want to give you a high-res version of my work is that I may want to make prints. As mentioned before, even the clients who buy up most of the rights to an image typically grant me the right to manufacture prints. I don't know about anyone else, but I charge for my prints. Not a lot, mind you — I'm not making limited edition giclees or anything. But, prints are part of my income. And being an artist, I need all the income I can get.
Were I to give you high-res scans of my work, what's to prevent you from making prints of your own? How do I know that you're not going to turn around and start selling prints, yourself? How do I know that you're not just going to keep passing along the files to other people? Quite frankly, I don't. There's no assurance that I won't be hurt by it.
Just BecauseLook, I've got decent sized images of my work available for all to see on various sites throughout the interwebs. My work has appeared in print and can be attained quite easily at game stores, online, and wherever books are sold. I've also been in shows all over the country where you could even see my work in person. You want to get up close and personal to it? There are plenty of opportunities.
I really don't care to speculate on your motivations for wanting the high-res file. Be it that you wish to design a desktop wallpaper, intend to undercut me on prints, want to claim it as your own work, or want to put it in your own project, it really doesn't concern me. Even if your intentions were pure, there's no telling what someone else's might be should you get hacked or have your files stolen.
I also don't care what the laws of your land happen to be, or what common practice might dictate in your culture (something that has come up in one or two of these requests). And you're right, I don't trust you. I don't even know you! Of course I'm not going to trust you. That this is a surprise or is met with any kind of indignation is a little odd to me. Nevertheless, the answer is still no.
Lastly, I don't care that you think me a jerk for refusing your simple request. I'm also not going to respond well to your pestering me for two weeks on the subject. Accept that it's just not going to happen. Please, just enjoy the images in the forms that are offered to you and stop asking for more. It doesn't become you. You're a better person than that.
At the end of the day, these images are a commodity. They're my commodity. And I'm not going to treat them lightly.