hI tried to go to the Postimg site a few more times, and still it wouldn't open. Then I searched for it on Yahoo (I had earlier done this on Google and the links to Postimg I found there didn't work either), and the link to its login page DID work!
I went to the site, and wondered whether I might have tried it before, perhaps a few years ago, so I attempted to login, and when that failed, I used the reset password procedure. It turns out I did already have an account, but hadn't uploaded anything. Possibly I tried out the service, but had trouble uploading at that time.
Here is the same image as before. Some of the urls Postimg offers don't work properly with this forum's image inserter button and field. The direct image url does:
PF reduces it to its maximum size, which is actually bigger than the normal image size stored by Picturetrail, the service I have been using.