TYPO3 Core

Did you try mod.wizards?
mod.wizards.newContentElement.wizardItems.common.elements.textpic.tt_content_defValues.imagecols = 1

Yes, that way it works. To be exact:
mod.wizards.newContentElement.wizardItems.common.elements.image.tt_content_defValues.imagecols=1

Jigal just brought that up. The TCAdefault-value is overridden by the wizard-default here (which is 2). He's already discussing internally how to proceed about this (kind of) "inconsistency".

How to test:

- TCAdefaults.tt_content.imagecols = 5 in user TSconfig
- make new CE with wizard of type "text"
- switch to type "image"
- on the Appearance tab the number of columns is 5

- make a new CE with wizard of type "image"
- on the Appearance tab the number of columns is 2

The 2 is set by mod.wizards.newContentElement.wizardItems.common.elements.image.tt_content_defValues.imagecols = 2 in pageTSconfig. This is used if you directly select the "image" CE type in the wizard. If you first select the "text" type imagecols is not set via defVals and the TCAdefaults setting is used.
Moving the imagecols=2 to the TCAdefaults for the default user TSconfig setting solves the problem without side effects (custom settings not working).

same story in TYPO3 4.7.5

But as I use templavoila (v. 1.7.0), I tried in pageTSconfig:

templavoila.wizards.newContentElement.wizardItems.common.elements.textpic.tt_content_defValues.imagecols=1
templavoila.wizards.newContentElement.wizardItems.common.elements.image.tt_content_defValues.imagecols=1

and it works perfect!

Patch set 2 for branch master has been pushed to the review server.
It is available at https://review.typo3.org/15118

Patch set 3 for branch master has been pushed to the review server.
It is available at https://review.typo3.org/15118

Patch set 4 for branch master has been pushed to the review server.
It is available at https://review.typo3.org/15118

Patch set 5 for branch master has been pushed to the review server.
It is available at https://review.typo3.org/15118

Patch set 6 for branch master has been pushed to the review server.
It is available at https://review.typo3.org/15118

Patch set 1 for branch TYPO3_6-1 has been pushed to the review server.
It is available at https://review.typo3.org/21980

Patch set 1 for branch TYPO3_6-0 has been pushed to the review server.
It is available at https://review.typo3.org/21981