Force images in theme options to use full uploaded size, not smaller size

[benlk]

HELPDESK-330 brought this up: Wordpress tries to use (and often succeeds in using) a scaled-down version of the uploaded image in the header image options in Appearance > Theme Options > Theme Images.

Two options:

• Wordpress should use an image that is at least as wide as the recommended image width for that file
• Wordpress should not use a resized image in that field at all

[benlk]

I don't know how this will be affected by switching to the Customizer for everything in #1438. I'm marking this as related, but this seems like something to not be exceptionally concerned about until we have the Customizer controls working.