How do I place a link to download a PDF file on an image?

I have my own image and I want to be able to click that image and download a pdf file.

First of all, the PDF needs to be hosted somewhere online. Then you need to insert the image in your jotform, as follows:

1. Click and drag the Image item from the Form Tools menu into the jotform, in the position where you want the image to appear:

2. Depending on if the image is in your JotForm account, somewhere else online or is yet to be uploaded, select the appropriate option, click Next and complete the wizard:

3. Once the image is on the jotform, click it and then click the Image Link button that's in the toolbar at the top of the form builder, insert the link to the PDF in the box and click OK:

4. Save the jotform

The above method will display the PDF in a separate tab when the user clicks the image, as demonstrated here. Did you specifically want the PDF to be downloaded instead of being displayed in the browser?