How can i download my picture from desktop to my i phone

There are several ways to transfer photos from a PC to iPhone:
Method 1: Use iTunes
Step 1: Install and launch iTunes on your PC. Connect your iPhone to PC.
Step 2: Click the iPhone button on the top-left corner of iTunes, and then choose Photos section.
Step 3: Check the box to Sync Photos.
Step 4: Choose either Pictures or Choose Folder in the drop-down menu.
Step 5: Select target photos to transfer or sync all to iPhone.
Step 6: Click Apply on the bottom-right corner.
Method 2: Use iCloud
Step 1: Open iCloud on your browser on PC.
Step 2: After log in your Apple ID, click the Photos icon on the main interface.
Step 3: Click the Upload button on the top-right of the website, and then choose photos from the local drive of your PC.
Step 4: Once you have selected photos, click Open to upload them to your iCloud Photo Library.
Step 5: If you have open the function of iCloud Photo Library on your iPhone, you will be able to see the uploaded photos on your iPhone's Photos app.

Method 3: Use third-party software
Step 1: Search and find a third-party transfer tool on Google, then download it. Here take iTransfer for reference.
Step 2: Connect your iPhone to PC, the software will recognize your iPhone automatically.
Step 3: Open Photos tab on the left sidebar.
Step 4: Click Add button to browse and choose photos on your PC. Then, click open to transfer.

Source: How to Transfer Photos from PC to iPhone?
Three Ways to Transfer Photos from PC to iPhone

It's very easy if you choose some third party software. You can just drag and drop photos from PC to iPhone 3S with iTransfer. The detail of steps refer below:
https://www.leawo.org/tutorial/how-to-transfer-videos-from-pc-to-iphone.html

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