[Plugin: AutoNav Graphical Navigation and Gallery Plugin] Warnings

That should be harmless, and should only occur when you access the administration screens. I will correct the offending code as follows (you may test this if you like, and let me know whether the warnings cease):

Immediately before line 113 in autonav-wl-options.php, which reads:

$wp_registered_sizes = array_unique(array_merge(array('thumbnail','medium','large'), $_wp_additional_image_sizes));

add the two lines:

$wp_registered_sizes = array('thumbnail','medium','large');
    if (is_array($_wp_additional_image_sizes))

Thanks for your reply. I will check the code that you suggested and I will let you know.

I just wanted to add that in this case the code only occurs in the admin screens but on other sites it appears on the actual site. For example on https://www.rewardcharts4kids.com/free-teacher-resources/
This is just one example.

I just tried the code and the warnings are gone on this site but on the other site https://www.rewardcharts4kids.com/free-teacher-resources/ (to which I added the code too) it seems like there are now even more warnings.

Right; see my response in your other thread. The rewards site errors are from something else entirely, perhaps caused by the directory structure on that web-host.