To sign a document on SeedLegals, you'll need to create a new signature, not upload an existing one.
This is for security reasons, so that we know it is really you signing.
But once you've signed, it will be saved and then you can use it for all your agreements on SeedLegals, in just one click.
And if you're asking to upload because it's not coming out well on your desktop, you might find it easier to perfect that sign on your phone / tablet with a touchscreen.
Want to learn more about electronic signatures, here is an interesting article: Are electronic signatures legally binding? How SeedLegals complies with UK law