According to what I'm reading on this, this is an E_WARNING, so it will just display at the top of the screen and upload the icon anyway (unless the forum's debug mode is activated). According to the history of PHP, this was added as a patchwork measure to deal with a bug in old versions of the cURL library, which has since been fixed, but the PHP warning was never removed.
A similar error will also occur if PHP Safe Mode is enabled in PHP < 5.4 (safe mode is removed by PHP 5.4).
This is fixed in the next release by not enabling the FOLLOWLOCATION option if safe_mode or open_basedir is set. The only drawback of this will be that external icons that have already been moved to a new location may break on future edits.