What you are describing here is the Splash Page.
The Splash Page is an image placeholder for the gallery which, when clicked, expands the gallery to fill the browser window.
The Splash page is displayed by default when the gallery is embedded in a web page alongside other content and displayed in Small Screen Mode (on mobile devices or when screenMode="SMALL"). The Splash Page can also be displayed by setting showSplashPage="ALWAYS".
Please see the Gallery Tour for more information about the Splash Page and Screen Modes.

If you are seeing the Splash Page in a desktop browser, then please ensure the following:
(1) Set screenMode="AUTO" (in JuiceboxBuilder-Pro's 'Customize -> General' section).
(2) Set showSplashPage="AUTO" (in JuiceboxBuilder-Pro's 'Customize -> Splash Page' section).
(3) Make sure that you do not have any browser extensions active that change the user agent string (to simulate other browsers) and also make sure that your browser is not in Responsive Design Mode.

Edit:
I've just realised that you have posted in the Juicebox-Lite forum and the configuration options in bold above are supported by Juicebox-Pro only.
Juicebox-Lite uses the default values of screenMode="AUTO" and showSplashPage="AUTO" (these values cannot be changed in Juicebox-Lite) so just check #3 above.

This sounds like it might be a different problem.
However, if you can resolve your original problem, then this may no longer be an issue.

Unfortunately, your screenshot does not show enough information to help me to troubleshoot your problems.
Please post the URL to your gallery's web page so that I can see the problems for myself and investigate further.
Thank you.

Please check your gallery's web page for HTML errors with the W3C Markup Validation Service.
When a web page is rendered differently in different browsers, this can often be attributed to HTML errors on the page.
Different browsers may respond differently towards errors (and some may be more tolerant towards errors than others).
Run your web page's URL through the W3C Markup Validation Service and fix any errors found.
Once the code on your web page validates correctly, your web page should be rendered with greater predictability and consistency across different browsers.

I hope my notes above help.
However, if you continue to experience difficulties, please post back with the URL to your gallery's web page so that I can see the problems for myself and hopefully help further.
Once I am able to see your gallery live on your web server, I should hopefully be able to determine the cause of your problems and propose a solution.
Thank you.