Has this occurred with your theme before? Theme pics/thumbnails not showing when logged in on Admin/Theme page above the Customize/Action prompts.
Sorry you are facing this issue, it would be my pleasure to help you.
Welcome to the Shopify community! 
Thanks for your good question.
Please share your site URL,
I will check out the issue and provide you a solution here.
Hi @MadamEstate ,
Please send your site and if your site is password protected, please send me the password. I will check it.