People can upload photos to my forum, this will be showed as thumbnails. But how can I remove this thumbnail (or photo) out of the post?.
By editing the post with the photo there is no line for it.
When you want to delete the thumbnail (photo) you had to delete the whole post.
Are you using standard MyBB Attachments?
Yes.
Modified in settings - posts
And what were the modifications? In a MyBB standard install then the ability to add, remove or update attachments is standard, so without knowing what you modified then it is difficult to help you.
Sorry,
I did not mean modified
I mark in settings - posts - people can add photo's that where showed as thumbnail.
And when i go to the admin page to view the attachments statistics I see a red cross
admin/index.php?module=forum-attachments&action=stats
Personally I can''t reproduce this. What browser are you using? Did you do any modification to your forum btw before you noticed this issue?