Image upload failures after server PHP update

[gsimon]

(Somebody with more insight will probably be along soon.)

The most likely cause of the 'headers already sent' warnings is whitespace after the closing tag (?>) in one of the php files. Start by looking at files that you may have edited – some editors will add a blank line at the end, which you definitely don't want.

You can remove that final php closing tag from the end of such files (the end-of-file implies the end of php) and then the problem can't recur.

Edit to add: What is the origin of that file 'sa_welcome.php' (shown in the warning message)? Is it legitimate or something that has been planted on the server?

Thanks!  I've begun searching for that blank.  To no avail so far.  However, do I understand correctly that the "?>" tag can be completely removed from files with no bad effects? 

The "sa_welcome.php" file appears to have been created by the original designer and my best guess is that it is a somewhat modified "welcome.php"  Unfortunately, there appears to be no way of contacting the original developer... 

In helping me find the possible white space can you/someone answer this:
In the error log output below, in which file might I best look for the white space?
=============Begin Log Output===============
[24-Apr-2017 14:57:10 America/Chicago] PHP Warning:  Cannot modify header information - headers already sent by (output started at /home/shabonna/public_html/gs/plugins/sa_welcome.php:12) in /home/shabonna/public_html/gs/admin/inc/cookie_functions.php on line 23
=============End Log Output================