First, you need to upload your photo to a photo cloud storage site like dropbox, shutterstock.com, imgbb.com, etc..
Imgbb (imgbb.com) is a good photo cloud site you can use.
Once you're on imgbb.com, go and upload your image first. Once the upload is complete, click the image itself:
On the image's page, scroll down a bit and open up the embed codes tab.
You'll see two sections which are for full images and thumbnails. You can copy the HTML code of the thumbnail if it's a logo.
Once copied, go into your campaign to which the form you want to place the image belongs, then edit it. Go to Step 2, and open up the Form Customization tab.
Go to the Form Footer HTML text box, and click on Source to toggle the Source code mode.
Once done, paste the code into the text box and save the step.
You can then check what it looks like by viewing the form.
NOTE - If using other image hosting sites besides Imgbb, you need to look for a way to grab the image's address.