When I get this, I put the Images page in a different tab to upload the image there, then put the name and file type (.jpg, .png, or .gif) of the picture in manually. That is usually what works for me, but this sometimes means that it will take up to 10 minutes to upload, depending on the file type, with .gif files taking longer than the others.
When I get the error, it's usually because my internet is running slowly, too many ads, or high traffic at the time, so the page I'm trying to directly load to can't get a good connection, so that's why I go to the image section of the wiki I'm on, because it can process a little faster. Still, it depends on the file type, but still, a slow internet connection can be a bit of a problem. If I can't get something to load, I can often send the image to another person via twitter or other sites and ask if they can upload it for me. It hasn't happened recently, but I've also added extentions to the browser I'm using (i.e. Adblocker) to allow less outside influences from slowling down my connection speed too much. If too many things are going on, my computer can't process it. I have an outdated version of Windows 10, but that hasn't stopped me from uploading, so being out of date isn't a factor in uploading pictures or accessing the site.
Well, I usually have several tabs working at once because I'm looking up stuff or working on a wiki, messaging someone, looking up pictures, recepies, and more. Because I am doing so much at once, my computer gets overloaded at times of high traffic, so having the Adblocker on to take off some of the load helps.
Based on #5, it sounds like the problem is intermittent.
With regards to what Fandyllic said, from what I can find online, there are some known rare cases when 503 is the result of an issue with the client's PC. The thing is that all the sites I look at that even mention this pretty much just say that it is so rare they aren't going to bother talking about it further. So yeah. If it is a problem with Hahie's PC, then I guess we are looking for a needle in a haystack.