I've read Sharepoint 2010 have a limit of the max number of items that can have a Item Level Permissions is 50.000.
This seems caused for bad performance above this number.
We're planning a project that has a picture library with about 300.000 items, and it's needed item level permissions.
What options I have to solve it?
You can split it to several web sites with picture libraries accordingly to image type or image meta information or user groups. That will limit number of files within one library. Also you can use folders for items with same permissions.
Please check this article for details about planning fine grained permissions from MS - http://www.microsoft.com/download/en/details.aspx?id=9030
I'd look closely at the rules to determine your permissions. If it's possible that you actualy have a few sets of permission levels look at splitting the library into a number of lists with differing permissions or even using folders with individual permissions and then placing your images into the required permission foilder and inheriting permissions.
