You can upload them to discord and then copy the link and post it in your account in the correct spot for it to work ^.^
The reason your image doesnt work in your signature right now is because:
The image linking on here is
![ ](LINK)
Your link must include https, your link has only http
But yes, an easier way to upload the image and getting a valid link to use is uploading it to discord first and then copying the image link ^.^ I hope this helps