CMS.SiteProvider.UserInfo ui = CMS.SiteProvider.UserInfoProvider.GetUserInfo("user");
CMS.TreeEngine.TreeProvider tree = new CMS.TreeEngine.Tree(ui);

//create the document that I want to attach the image to:
TreeNode n = new TreeNode("documentType", tree);
TreeNode p = tree.SelectSingleNode("website", "article/path", CMS.TreeEngine.TreeProvider.ALL_CULTURES, true, null, false);

n.DocumentName = "foo";
//omitted: code that adds the data to the document

//insert the document
CMS.WorkFlowEngine.DocumentHelper.InsertDocument(n, p.NodeId, tree);

//read the image from the other database:
Binary file = (from img in _db.images where img.id == imageId select a.data).SingleOrDefault();

//create an instance of a HttpPostedFile, see bottom for the ConstructHttpPostedFile method
HttpPostedFile postedFile = ConstructHttpPostedFile(file.ToArray(), Guid.NewGuid().ToString() + ".jpg", "image/jpeg");

//create the attachment
AttachmentInfo ai = null;
ai = CMS.WorkFlowEngine.DocumentHelper.AddAttachment(n, "guidColumnName", postedFile, tree);
//an alternative without the HttpPostedFile would be:
//ai = CMS.WorkFlowEngine.DocumentHelper.AddAttachment(n, "guidColumnName", "", tree);
//however that's also not working
ai.AttachmentMimeType = "image/jpeg";
ai.AttachmentBinary = file.ToArray();
ai.Update();
n.Update();

//method from http://stackoverflow.com/questions/5514715/how-to-instantiate-a-httppostedfile
public HttpPostedFile ConstructHttpPostedFile(byte[] data, string filename, string contentType)
{
    // Get the System.Web assembly reference
    Assembly systemWebAssembly = typeof(HttpPostedFileBase).Assembly;
    // Get the types of the two internal types we need
    Type typeHttpRawUploadedContent = systemWebAssembly.GetType("System.Web.HttpRawUploadedContent");
    Type typeHttpInputStream = systemWebAssembly.GetType("System.Web.HttpInputStream");

    // Prepare the signatures of the constructors we want.
    Type[] uploadedParams = { typeof(int), typeof(int) };
    Type[] streamParams = { typeHttpRawUploadedContent, typeof(int), typeof(int) };
    Type[] parameters = { typeof(string), typeof(string), typeHttpInputStream };

    // Create an HttpRawUploadedContent instance
    object uploadedContent = typeHttpRawUploadedContent
      .GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, uploadedParams, null)
      .Invoke(new object[] { data.Length, data.Length });

    // Call the AddBytes method
    typeHttpRawUploadedContent
      .GetMethod("AddBytes", BindingFlags.NonPublic | BindingFlags.Instance)
      .Invoke(uploadedContent, new object[] { data, 0, data.Length });

    // Create an HttpInputStream instance
    object stream = (Stream)typeHttpInputStream
      .GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, streamParams, null)
      .Invoke(new object[] { uploadedContent, 0, data.Length });

    // Create an HttpPostedFile instance
    HttpPostedFile postedFile = (HttpPostedFile)typeof(HttpPostedFile)
      .GetConstructor(BindingFlags.NonPublic | BindingFlags.Instance, null, parameters, null)
      .Invoke(new object[] { filename, contentType, stream });
    
    return postedFile;
}

CMS.SiteProvider.UserInfo ui = CMS.SiteProvider.UserInfoProvider.GetUserInfo("user");
CMS.TreeEngine.TreeProvider tree = new CMS.TreeEngine.Tree(ui);

//create the document that I want to attach the image to:
TreeNode n = new TreeNode("documentType", tree);
TreeNode p = tree.SelectSingleNode("website", "article/path", CMS.TreeEngine.TreeProvider.ALL_CULTURES, true, null, false);

n.DocumentName = "foo";
//omitted: code that adds the data to the document

//insert the document
CMS.WorkFlowEngine.DocumentHelper.InsertDocument(n, p.NodeId, tree);

//read the image from the other database:
Binary file = (from img in _db.images where img.id == imageId select a.data).SingleOrDefault();

//save the image to disk
string fileName = Guid.NewGuid().ToString() + ".jpg";
FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.Write);
byte[] b = file.ToArray();
fs.Write(b, 0, b.Length);
fs.Close();

//create the attachment
AttachmentInfo ai = null;
ai = CMS.WorkFlowEngine.DocumentHelper.AddAttachment(n, "guidColumnName", fileName, tree);
ai.AttachmentMimeType = "image/jpeg";
ai.AttachmentBinary = file.ToArray();
ai.Update();
n.Update();

//delete the image
File.Delete(fileName);