Microsoft Acquisition
iView MediaPro is now Microsoft Expression Media. PictureSync supports Expression Media, MediaPro 2.6.x, and MediaPro 3.0.1+ (early builds of 3.0 are not compatable).
Tutorial
iView have produced a helpful guide to uploading with PictureSync to Flickr (also applies to other services), if you're new to using PictureSync with iView download the PDF (1.4 MB).
Usage of PictureSync with Expression Media requires a PictureSync Pro licence, otherwise although there are no functional limitations you will be shown a purchase reminder upon each use.

Opening Pictures

Select some images in your catalog, launch PictureSync and click the import/open button if necessary (when you launch PictureSync it automatically opens the selected pictures). This will import the annotations saved in your Expression Media catalog as opposed to those saved as IPTC/XMP fields in the file, and will enable resizing/watermarking using MediaPro (if that option is turned on). Do not control-click and use 'Open With' or drag and drop as PictureSync will be unable to rerieve Expression Media's metadata (unless saved as IPTC/XMP).

Image Conversion

You can have PictureSync resize your images using MediaPro upon upload. To activate resizing:

The maximum size settings in PictureSync are not used, instead a Convert Image resizing options pre-set in your ~/Library/Application Support/Expression Media folder is used. With resizing enabled, after clicking save in the Settings dialog you will be asked to select an options pre-set for that account. You can change the image reszing options and create new ones in Expression Media:

Rotation

PictureSync will respect the rotation set in Expression Media and perform a lossless rotation at upload if it does not match the image. When using Expression Media, PictureSync does not respect the camera/EXIF orientation setting as Expression Media does this itself when you add an image to a catalog.

AppleScript

You can use AppleScript to select items in your catalog that have been uploaded with PictureSync. You could for example set a custom field value so that you can more easily identify uploaded items from your catalog. Download example script. You can run the example script from Script Editor, or place it in your ~/Library/Expression Media/Plug-ins/Scripts folder. PictureSync can also perform uploads using AppleScript.