Are you sure you are doing it properly? To submit a thumbnail with your figure, ensure that you attach it to the submission email. On Samsung devices, you’ll find a paper clip icon for attachments, while on iPhone, look for an image icon on the keyboard to add the file.

You’ve uploaded figures before, so I’ll assume you know how to do it.

Try saving the email as a draft and opening it in your mail app. You might find the option to add an image there.