I have posted a complete solution of what worked for me on Emacs SE:
http://emacs.stackexchange.com/a/2458/115

I modified the image-type-header-regexps list so that imagemagick is used
to more more than just jpg files.


--
Kaushal Modi

On Wed, Oct 22, 2014 at 2:50 PM, Kaushal <kaushal.modi@gmail.com> wrote:

> I went deeper in the references bug posts and I found the answer in bug #
> 10112!
>
> I had to add:
>
> (add-to-list 'image-type-header-regexps '("\\`\377\330" . imagemagick))
>
> But just for my knowledge, what is "\\`\377\330" ?
>
>
>
>
> --
> Kaushal Modi
>
> On Wed, Oct 22, 2014 at 2:45 PM, Kaushal <kaushal.modi@gmail.com> wrote:
>
>> Thanks for your reply.
>>
>> I evaluated (setq image-type-header-regexps nil) and verified that it is
>> actually set to nil by doing C-h v on that variable.
>>
>> But (eq image-type 'imagemagick) is still returning nil and I am M-:
>> (image-transform-set-scale 0.1) still does nothing.
>>
>>
>>
>>
>> --
>> Kaushal Modi
>>
>> On Wed, Oct 22, 2014 at 1:52 PM, Glenn Morris <rgm@gnu.org> wrote:
>>
>>>
>>> This message was over 1 MB. Please compress attachments in future.
>>> No need to resend this one, since interested people can fetch the
>>> attachment
>>> from http://debbugs.gnu.org/cgi/bugreport.cgi?bug=18797#5 .
>>> (But I think it has no relevance anyway.)
>>>
>>> Kaushal wrote:
>>>
>>> > I have compiled emacs with Imagemagick (the config.log file is
>>> attached).
>>> >
>>> > But "M-x image-transform-set-scale" is not working for me. The image
>>> scale
>>> > stays the same when I set the scale to 0.5. I tested it with bot png
>>> and
>>> > jpg images.
>>> >
>>> > But the eimp package is working fine (which uses mogrify from
>>> Imagemagick).
>>> >
>>> > When an image is open in a buffer, evaluating "(eq image-type
>>> > 'imagemagick)" returns "nil".
>>>
>>> Right, so it's not an ImageMagick image, so you can't transform it.
>>> Emacs uses libjpeg for jpgs by default.
>>> Try testing it on a format that Emacs does't support otherwise, eg bmp.
>>> You have to force the images to be created with type imagemagick to be
>>> able to scale them. This remains poorly documented IMO;
>>> http://debbugs.gnu.org/10746
>>>
>>> > What am I missing in enabling the imagemagick detection for functions
>>> like
>>> > image-transform-set-scale?
>>>
>>
>>
>