When using a slideshow module, we upload the images to WordPress, once saved we get a blue question mark instead of the uploaded image. You wont be able to see the error because we are working on it on our staging website. On my MacBook it shows as blue square with a ? mark. On windows it shows as blank like I attached below. Hope you can help.
Tip: From your description it seems the browser is not able to find and load the image. In that case you can check your website for any console errors (just open the console in your browser and refresh the website to see if any errors displays in console). That way you will be able to trace and potentially resolve the issue.