Is there a way for me to re-upload this or edit it to get the bigger, better picture? Fairly new to uploading here so I don't have much of an idea.
On the Pixiv page with the image descriptions, tags, etc., click on the image itself. This will open the original-sized image. Right click this, copy the image location (not the URL in the address bar), and paste it into the source box on the Danbooru upload page. If it doesn't end with a file extension (.jpg/.png/etc.) then you've done it wrong. EDIT: If it's one of those multi-page manga viewer thingies, a small button will appear at the top left of each image that looks like two arrows pointing diagonally away from each other. Clicking that will get you the original size; these specific image URLs have "_big_p#.xxx" at the end indicating that they're the "big" version and their page number.
That'll pull the image straight from Pixiv in its largest format and fill in the source all at once.
Should I just upload another one with a bigger image or is there a way to slot in the new one into the same post?
Upload the "proper" one as above, then add its post number as this post's parent (under the Edit menu, where the tags are changed from). It's likely this one will be deleted, but that's okay.
