If you want to allow all authenticated users to be able to upload images in the forums, go to Administration > Permissions > Roles that can browse and upload but only in a user specific folder.
Allow the role "Authenticated Users". Or if you want to limit it to only some users then use a custom role and add the chosen users to the role.
There can be security concerns any time you allow users to upload files so it is up to your own judgement whether to allow strangers to upload files.
Hope that helps,