Sorry, the Reproduce code would be:
---------------
<?php

$txt = "Next: <a
href=\"http://www.joelonsoftware.com/uibook/chapters/fog0000000063.html\
" target=\"_blank\">Designing for People Who Have Better Things To Do
With Their Lives, Part Two</a> <br />????<br />????<img
src=\"http://86.0.190.20/test/ipb21/skin_acp/IPB2_Standard/images/users.
png\" border=\"0\" align=\"absmiddle\" alt=\"????ͼƬ\"
onload=\"if(screen.width*0.7<this.width) {this.resized=true;
this.width=screen.width*0.7;}\" /><br />????<b>Adverment&#33;</b> Do you
need to control a computer remotely, even when firewalls get in the way?
My company&#39;s latest product, <a href=\"https://www.copilot.com/\"
target=\"_blank\">Fog Creek Copilot</a>, is a remote control system that
requires no setup, no configuration, and works even if both users are
behind firewalls. It&#39;s designed to make remote tech support easy.
<br />????<br />????Enter your email address to receive a (very
occasional) email whenever I write a major new article. You can
unsubscribe at any time, of course.<br />????</div>";

$txt = strip_tags( $txt );
print $txt;

?>

Can't reproduce.
Both your scripts work fine with any PHP version I can find (4.3.11, 4.4, 5.0.x, 5.1).

I tested with 4.3.11, It also have the bug.

------
<?php

$txt = "Next: <ahref=\"http://www.joelonsoftware.com/uibook/chapters/fog0000000063.html\" target=\"_blank\">Designing for People Who Have Better Things To DoWith Their Lives, Part Two</a> <br />????<br />????<imgsrc=\"http://86.0.190.20/test/ipb21/skin_acp/IPB2_Standard/images/users.png\" border=\"0\" align=\"absmiddle\" alt=\"????ͼƬ\"onload=\"if(screen.width*0.7<this.width) {this.resized=true;this.width=screen.width*0.7;}\" /><br />????<b>Adverment&#33;</b> Doyouneed to control a computer remotely, even when firewalls get in theway?My company&#39;s latest product, <a href=\"https://www.copilot.com/\"target=\"_blank\">Fog Creek Copilot</a>, is a remote control systemthatrequires no setup, no configuration, and works even if both users arebehind firewalls. It&#39;s designed to make remote tech support easy.<br />????<br />????Enter your email address to receive a (veryoccasional) email whenever I write a major new article. You canunsubscribe at any time, of course.<br />????</div>";

$txt = strip_tags( $txt );
print $txt;

?>
-----------

It print "Next: Designing for People Who Have Better Things To DoWith Their Lives, Part Two ????????", but not the whole html tags striped string.

OS:  Windows XP
PHP: 5.0.3

Further investigation:
----------------------
strip_tags does not handle tags correctly if '<' is occurring in an attribute value as it does in the example in the onload="if(screen.width*0.7<this(...)" attribute

strip_tags does provide for nested tags as shows the following example, but doesn't ignore quoted < and > inside tags as the following two examples show:

<?php
//example 1
$txt = 'text1<tag attr="<"> text2<tag attr=">"> text3';
$txt = strip_tags( $txt );
print $txt;
// prints 'text1 text3'
// should print 'text1 text2 text3' 

//example 2
$txt = 'text1<tag <nested tag>> text2';
$txt = strip_tags( $txt );
print $txt;
// prints 'text1 text2' as it should if strip_tags searches for nested tags
?>

Two questions arise:
1. Is the detection of nested tags reasonable?
2. Is a HTML-tag malformed if it contains a quoted '<' or '>' sign?
3. If such a HTML-tag is malformed, shouldn't strip_tag() still work correctly, since there is no ambiguity thanks to the quotes?

Answers:
1. I cannot figure any real world example of nested tags other than HTML-comments like <!--<br />-->
2. According to w3c.org html-validator, it is valid.
3. Strip_tags should still work

No parsing of quotes:
---------------------
The following example proves, that strip_tags does not parse quotes inside '<' and '>' at all
<?php
//example 3
$txt = 'text1<tag attr="> text2 <tag attr="> text3';
$txt = strip_tags( $txt );
print $txt;
// prints: 'text1 text2 text3'
// should print: 'text1 text3' since there is one only tag with an attribute 'attr="> text2 <tag attr="'
?>

Conclusion:
-----------
This IS a real bug, but not related to <img> tag, but to not parsing of attributes and quotes inside tags.

It would be important to correct that bug

With best regards Lorenz Meyer

Warning	
Because strip_tags() does not actually validate the HTML, partial, or broken tags can result in the removal of more text/data than expected.
(c) http://php.net/strip_tags