I don't know when or why it happened, but I am only getting a red X where the image should be.
When I did the test (ubbthreads.php?ubb=captcha&t=im&test=1) suggested by Rick, I received the following error message:
PHP error: sh: /home/maritim5/www/forums/images/captcha/backgrounds/background1.jpg: cannot execute binary file
I am still using the 7.4 final version.
Any thoughts on what is happening on my end?