`IkiWiki::Plugin::img` appends `[$pagenumber]` to the filename to deal with multipage documents such as PDFs.
However, `Image::Magick` doesn't seem to like page selection for filenames containing a colon.  This is also the case for imagemagick binaries:

    $ identify 'screenshot_2015-06-06_18:37:53.png'
    screenshot_2015-06-06_18:37:53.png PNG 453x122 453x122+0+0 8-bit sRGB 11.2KB 0.000u 0:00.000
    $ identify 'screenshot_2015-06-06_18:37:53.png[0]'
    identify: no decode delegate for this image format `37' @ error/constitute.c/ReadImage/501.
    $ mv 'screenshot_2015-06-06_18:37:53.png' 'screenshot_2015-06-06_18-37-53.png'
    $ identify 'screenshot_2015-06-06_18-37-53.png[0]'
    screenshot_2015-06-06_18-37-53.png[0]=>screenshot_2015-06-06_18-37-53.png PNG 453x122 453x122+0+0 8-bit sRGB 11.2KB 0.000u 0:00.000

This might be an imagemagick bug, but it's also possible that colons
are interpreted somehow.

> Yes they are: ImageMagick has syntax to force the file to be
> interpreted as a particular format, like gif:foobar.img to
> read foobar.img as a GIF, or to use unusual I/O patterns, like
> fd:5. --[[smcv]]

Anyway, to render such images properly in ikiwiki I had to remove
the colons.  An easy fix is to remove ‘:’ from `wiki_file_chars`,
but this can break existing installations.

> I think we should remove `:` from the default `wiki_file_chars`
> either as soon as we have a way to handle backwards compatibility,
> or in ikiwiki 4; but you're right that it's a compat problem,
> so we can't just do that as a solution. --s

A better solution would be to make `IkiWiki::Plugin::img` croak on
such image filenames (which anyway are currently not rendered, but
`Image::Magick`'s error message is quite cryptic).

> Better still would be to fix the bug by escaping the filename
> so ImageMagick treats it as just a filename. It seems the way
> to do that is to call `Read(":hello:world.png")` instead of
> `Read("hello:world.png")`, which I have now [[done]]. --s
