When creating a Smartlink, we are getting as much as possible automatically the image from the source element you used (when selecting it from Soundiiz loaded library or with the URL).
While we are not proposing "upload" button for images, you can still update the Smartlink image in the "Edit" options of the Smartlink.
We are recommending you to host yourself your image to be sure to always have it online or using an online image storage provider. Checkout https://imgur.com/ for example (be sure to set your image as private).
Getting "invalid" message when pasting your Image URL ?
Be sure to check security measures in your browser / try with another browser as your image could not be accepted if using http:// instead of https:// for example.