You can easily insert an image into a PDF document, thanks to a free PDF editor available online.
Steps

Step 1. Go to SmallPDF
This free tool lets you open a PDF file in your browser and add your own data, including images.

Step 2. Click Choose file
The button is in the center of the page, in the blue box. This will launch the file explorer in your browser.

Step 3. Find your file
Go to the folder containing the PDF you want to edit. The file you are looking for must have the.pdf extension.

Step 4. Select the file
Click on the file to select it and press the Open button. The file will open in Smallpdf and be ready to edit.

Step 5. Click ADD IMAGE
This is the second option in the edit menu, just above your document window.

Step 6. Find your image
Go to the folder that contains the image to insert. You can insert most image formats, such as JPG, GIF, and PNG.

Step 7. Select your image
Click on the image to select it and press the Open button. Your image will then appear in the PDF file.

Step 8. Edit the image
Change the size of the image by dragging its corners until it reaches the desired size.

Step 9. Position the image
Drag the image to where you want to insert it. Click anywhere on the image, then, while holding the click, drag it to the desired location.

Step 10. Click Finish
The button is located in the lower right corner of the page. This will save your document and redirect you to a page with a download link.

Step 11. Click Download File
This will start downloading the file to your computer.