Welcome to the community @0xDon
I’m not too familiar with the Discord API, but I do not think you can do polling since the API that allows you to detect changes is socket base. I think we will need to create a Discord Trigger to make this workflow possible. There is already a feature request to create the Discord Trigger. Make sure you upvote it.
Create tweet seems to have a section for attachments but not sure how it works if I just have a link to the image
As long as you can download the data using the link, that should not be an issue. You can download the data using the HTTP node and then reference the image (binary data) in the Twitter node using the attachment field.