image files are deleted after some time

hi

I am uploading some avatar images, everythink seems okay BUT after some time those files are deleted from site/default/files/avatar.....

- at this moment I am using stable release but I did also try dev release, still same problem
- I did just upload images and doing nothing with site {it is new project and there are no another users} : same situation

I did run out of ideas what can be wrong: persmision, directories existing ...
have you got any tip what should I check?