Updated images

[flukeout]

Follow up to https://github.com/mozilla/id.webmaker.org/pull/432

• Mozilla logo in header no longer appears in all black in Firefox
• Split the checkmark image into two separate images because...
  • It needs to be green for the email opt-in checkbox
  • It needs to be white near the Terms of Service links

Should look like this...

cc @cadecairos

[flukeout]

Thanks @cadecairos