public int GetTagUsageCount(int tagID)
{
    using (var db = _databaseManager.CreateDBManager())
        return
            db
                .PictureTagRelations() //возвращает таблицу
                .Where(r => r.TagID == tagID)
                .Count(r => r.IsAccepted == true && r.Picture.IsAccepted == true && r.Picture.ReplacementID == null); //Picture - ассоциация
}

'r.Picture.IsAccepted' cannot be converted to SQL.