Perhaps I'm going to sound like an idiot, but I'm a little frustrated right now.
So I uploaded an image, 1200 x 1920. The upload seemed to take a very long time, and when it finally finished after 10 minutes (much longer than an upload of an image that size normally takes), I found myself at an error page! Puzzled, I went to my profile and found that the image had in fact made it onto the site.
I visited the image page to find that the image had uploaded, but only at a size of 600 x 960! Despite this, the image information to the left claimed that the resolution was 1200 x 1920. So, seeing that the upload had gone wrong and somehow corrupted, I decided to delete the image and reupload it. Well, stupid me, it was only too late until Danbooru told me that I couldn't upload the image once more, because it was a duplicate!
I find it very puzzling indeed that once an image has been deleted from the site by the person who uploaded it, no one is able to upload it again for any reason.
Updated by user 9546