I had the same problem. It started today. My browser told me that it couldn't find the postimage page as it was listed on my bookmark. I started over by retyping
www.postimage.org to my browser and was able to access the site. I got a message stating that they had been unexpectedly dropped by what I assume is their web service (I didn't totally understand the terminology) and that they had reopened with another web address code which works because a half our ago I successfully posted pics on another website. However my older pics no longer show up there. The other part of the post image message was that you will need to update all of the links to pictures you have posted in the past. For now, I am going to stick with them because this may be a situation beyond their control and since I like the simplicity of their service, I will stick with them.