Apache returning text/html on some png files

Did you make sure png files are valid image files? If you are on a Linux platform you can try the file command

file somefile.png

That command should return something along the lines of

PNG image data, 318 x 15, 8-bit/color RGB, non-interlaced

You can also try to view the file with a image viewer.

The problem for me was that the image file to be delivered was named pear.php.net-sos8j3lis2j.png.

Apache's AddHandler directive - that's used to assign the PHP interpreter to .php files - supports multiple extensions, and pear.php.net.png was seen as having three extensions: .php, .net and .png. .php was the first, to the php interpreter was invoked.

text/html was then sent out by either apache or PHP, I don't know.

What I had to do to fix the problem:

Replace all instances of

AddHandler php-cgi .php

with

<FilesMatch \.php$>
    SetHandler php-cgi
</FilesMatch>