Just downloaded a v2.5 for first time use and get stuck also with the uploading of image. Don’t have any plugins yet.
Even it says that the uploading has failed, the image is uploaded into my library. So to be able to post an image, I then go to libray, click on the permalink of the picture, then get the URL of the image, copy anf paste into the poste.
It’s a painful process just to post an article, but can bypass the problem by the time being.