Strange behavior with pictures

Strange behavior in my multi-site with pictures. When i upload a picture (in article or media-library) it will be saved in the different sizes, i´ve checked it via ftp
– but… when i wanted to pick this picture in an article i could only choose the full size picture – no minimal size, no medium size… – in the last 250 articles of the blog, i´ve no problem to choose the right Image size, but now…

Any solution will be great!