I have a large number of pictures in my iPhone but the memory of iPhone is limited, I need free the space to save more data. So how do I transfer pictures from iPhone to pc?

Either bluetooth transfer or via usb cable.

Follow the below steps to transfer pictures from iPhone to
your PC.

Import photos or video from your device to your PC:

1. Connect
       your iOS device to your computer. If the AutoPlay window appears, click
       "Import Pictures and Videos
       using Windows,” If the "Import Pictures and Video" dialog
       appears, skip to step 4.
       If the AutoPlay dialog doesn't appear, then follow the below link.
       http://support.apple.com/kb/TS1500
2. Choose
       Start > Computer.
3. Right-click
       the logo for your device and click "Import pictures and videos."
4. Click
       Import Settings.
5. Click
       the Browse button by the "Import to" field and choose where to
       import your Camera Roll's photos. By default, these will import to the My
       Pictures folder.
6. Select
       "Always erase from device after copying."
• If
        you delete photos after you import them, you can reduce the size of your
        Camera Roll and improve backup and restore times.
• If
        you want to import the photos to additional computers, don't select this
        option.
• If you
        don't want Windows to delete photos from your camera on subsequent
        imports, remember to come back here and deselect this option when you're
        finished.
7. After
       setting your import options, click OK to dismiss the window.
8. Choose
       a tag and click Import.
9. After
       the import is complete, Windows Photo Gallery will show your photos and
       videos in Recently Imported.

If some of your photos didn't import from your device's
Camera Roll, you can copy the photos to your computer using Windows Explorer or
import the photos using software such as Windows Live Photo Gallery or Adobe
Photoshop Elements 3.0 or later.

Copy the photos to your computer using Windows Explorer:

1. Connect
       the iOS device to the computer.
2. Choose
       Start > Computer.
3. Double-click
       the logo for the device to browse its folders.
4. Go to
       [iOS device]Internal StorageDCIM100APPLE.
5. Copy
       the content in this folder to your computer.
6. Disconnect
       your device. Open the Camera Roll and delete the photos you imported.

To copy the photos to your computer using Windows Live Photo
Gallery or Adobe Photoshop Elements 3.0 or later, see the documentation for
that application.