How to link an image to an item in Item Master in AX 2012?
1) Copy the images in the required  path.
2) Create a new job to link images to items.
   static void uploadProductImages(Args _args)
{
    DocuRef                         docuRef;
    DocuValue                       docuValue;
    InventTable                     inventTable;
    EcoResProductImage       ecoResProductImage;
    System.String[]                 fileNames;
    int                                     fileCount, i;
    str                                     fileName, trimmedFileName, fileNameWithExt;
    BinData                            binData;
    str                                     filePath;
    System.IO.FileInfo            fileInfo;
    EcoResProduct                 ecoResProduct;
    EcoResProductImageManagement    productImageManagement;
    EcoResProductImageThumbnail       ecoResProductImageThumbnail;
  
    filePath = 'Specify the path where your images are saved here.';
    // File path should be provided as : C:\\Images\\Car.jpg
    binData  = new BinData();
    fileNames = System.IO.Directory::GetFiles(filePath);
    fileCount = fileNames.get_Length();
    ttsBegin;
    for (i=0; i<fileCount; i++)
    {
        fileName = fileNames.GetValue(i);
        // Get only the file name. If value returned is C:\Images\Car.jpg, get only Car.jpg
        trimmedFileName = substr(fileName, strscan(fileName, '\\', strlen(fileName),
        -strlen(fileName))+ 1,    strlen(fileName));
        fileNameWithExt = trimmedFileName;
        if (trimmedFileName)
        {
            // Assuming file extension is always .jpg, removing it
            trimmedFileName = strreplace(trimmedFileName, ".jpg", "");
        }
        // assuming image name matches item name in AX
        ecoResProduct = EcoResProduct::findByDisplayProductNumber(trimmedFileName);
        inventTable = InventTable::findByProduct(ecoResProduct.RecId);
        if (inventTable)
        {
            binData.loadFile(fileName);
            docuValue.FileName = trimmedFileName;
            docuValue.FileType = "jpg";
            docuValue.OriginalFileName = fileNameWithExt;
            docuValue.File = binData.getData();
            docuValue.insert();
            docuRef.ValueRecId = docuValue.RecId;
            docuRef.RefTableId = tableNum(InventTable);
            docuRef.Name     =  trimmedFileName;
            docuRef.RefRecId = inventTable.RecId;
            docuRef.RefCompanyId = inventTable.dataAreaId;
            docuRef.TypeId = "Image";
            docuRef.insert();
            ecoResProductImage.clear();
            ecoResProductImage.DefaultImage = true;
            ecoResProductImage.FileName = fileNameWithExt;
            ecoResProductImage.ImageFormat = "jpg";
            ecoResProductImage.RefRecId = docuRef.RecId;
            ecoResProductImage.RefRecord = docuRef.RefRecId;
            ecoResProductImage.Usage = EcoResProductImageUsage::External;
            //0; // Base Enum: 0=External, 1=Internal
            // To make the thumbnail size
            ecoResProductImageThumbnail  = new EcoResProductImageThumbnail(false);
            ecoResProductImage.MediumSize =                       ecoResProductImageThumbnail.generateThumbnail(204,204,docuRef);
            ecoResProductImage.ThumbnailSize    = ecoResProductImageThumbnail.generateThumbnail(48,48,docuRef);
            ecoResProductImage.insert();
            info(strFmt("%1 - %2", docuValue.RecId, docuRef.RecId));
         }
      }
    ttsCommit;
}
3) Make sure you have a document type of  type "Image"(as we are linking images to items)  created  in
    Organization Administration > Setup > Document Management > Document Types.
Thanks & RegardsSindhu