I encountered this problems for a lot of times since the past days. Whether I am uploading images in my blog or posting the articles that needs to be posted, this error 503 Service Unavailable is always appearing. What I do when I encounter is simply to try and try again until I am successful.
I browse online what are the causes of this and I found very important information about it. Blogger Tricks for example has an explanation why this error is being encountered.
“Who is affected by this 503 “Service Unavailable”
This problem affects all blogs worldwide. One of the ways to find out if you have been affected is to login in Google analytics and check your traffic sources, select search engines and select the keyword that brings most of the traffic to your blog, compare the data with previous months. A notable change in your bounce rate or average time will be a good indicator that your blog had been affected by this 503 response code. The logic behind this is simple, a user trying to view your blog has opened the 503 page and assumed the site down and gone to another site. Due to this you will receive a 100% Bounce rate for that keyword as well as a low time average.
Understanding the problem and its consequences
The consequences of this problem are that you can start to lose traffic and your ranking in search engines. You need to understand that this is not a problem of resources, this cannot be fixed by getting a new server for blogger (if it was I would not be here talking about it).
This is a problem of how we blog and what we do with our blogs.”