1. Find the image you want
2. If the image is on Tumblr, you can right-click with your mouse and copy the image address. If it's on a site like Pinterest, you'll need to put it in an image sharing site. A lot of people use imgbb or catbox.moe because they're free and easy.
After you upload it to imgbb/catbox, copy the image address.
3. In your "edit profile" text box, type this:
<img src="https://ADDADDRESSHERE"/>
Click "save", then refresh your profile.
Hope this helps! Let me know if you have issues.