I experienced this very frustrating problem for several weeks, but despite repeated support requests (not from this address), I never learned of a response or a solution.
My problem sounded identical to yours in that suddenly half of the images stopped loading properly on my site. If I hit 'refresh' then they all showed blank thumbnails. If I reloaded the site, some of the images (usually different ones) would load OK, but the rest would be blank again.
The interesting thing was that the problem started for me immediately when I took up the domain mapping to move my blog from .wordpress.com to .com - and resolved itself immediately when I put it back to the .wordpress.com address.
The domain mapping option appears to be a simple redirect operation, but it's not actually complete, since if you check out the img src addresses you'll see that all your images remain at a .wordpress.com address.
The redirect process associated with the domain mapping seemed to manage with text, but somehow it just wasn't up to reliably redirecting the images across. It simply didn't work for me. I read here and there of server problems, and that the problem would be solved, in time, but so far I don't believe that it really has.
Originally I thought that the problem had to do with the theme I'd selected for my site, just as you thought it was to do with your blogdesk uploading. But finally I concluded that it had nothing to do with that, but was simply a problem at WordPress - nothing more than that, just a simple glitch that needs to be fixed by WP, but hasn't been, yet.
I sent a lot of feedbacks about the problem, but never got any reply. All my other issues were solved more or less instantly, but not this one, and those feedbacks just disappeared into the ether without trace.
That was very frustrating, too, since it's not clear how to resolve a problem here if not through that route. What do you do ? Pick up the phone and call someone at Automattic ? But who ? No, I didn't - it's a free service after all, so it would be hard to jump up and down and make a fuss.
In some ways, the $15 I spent on domain mapping was a complete waste of cash. I started off with a good-looking blog and ended up with a really amateur-looking one. That was a real downgrade, and not the upgrade I had hoped for, so I just had to go back to the original.
But, looking on the bright side, I do now at least own the .com address for my site, and hits to that url now redirect back to the old .wordpress.com address, which has more or less the same effect I wanted, although it's not quite so pretty.
And, given time, I would expect WordPress to be able to solve the problem, so that the .com address will become useable eventually, I hope, maybe once they've got their servers into shape.
They've made everything else work very well around here, after all, and overall I think the service works well.