I have some spritesheets that are already on the site but I have more to add/errors to fix. I see how to submit new sheets, but not editing ones. What should I do?
Also where are the blank sheet icons?
Go to the sheet you want to edit, at the top-right of the info box there should be a "Revise" link. Click this and you can modify the sheet, icon, or various other things. Like submissions, these need to be approved by staff before they appear on the site.
As for icons, you simply need to submit a 148x125 image for the icon and the border/background is automatically generated by the site itself.
See the wiki page.
You don't. You just need to make a transparent 148x125 PNG in Photoshop or GIMP.
Edit:
Moved this post here because I assume this is what you were trying to reply to originally.
-Petie