It is easy to edit a PDF with Mac OS X preview app. Especially to fill up a PDF form. You can also paste your signature on it.

But the constraint is to paste an image (e.g. your photo or company stamp on it)

I found this free tool online that can do the job.

https://www.sejda.com – no signup but limited uploads and feature for free package

There are many others, but this one is the easiest, fast and straight forward.

If the link is not working in the future, you can always google for it.