As I was composing the previous entry, I ran into a problem uploading the accompanying image. While the upload dialog popped up, red x marks were shown instead of file icons. I was able to choose the file, but the upload did not begin. I had to manually do it using FTP.
I thought it was a Java-plugin problem, so I thought I install the necessary plugin first, then would update the post after the problem is resolved. I guess I was wrong in thinking it had something to do with Java, as it appeared to be a Flash-plugin issue, as discussed in this bug report on LaunchPad. As one of the commenters said, this problem “is present on 64-bit hard drive installations,” such as mine.
For now, I could only search for possible solutions, or use FTP for image uploads while waiting for the experts to resolve this issue.