The 'assets.png' file was a resource file used to display gallery icons in older versions of Juicebox. It was removed from v1.3.0 when Juicebox introduced vector icon fonts for button icons. Please see the Using Custom Icons support section for further details.
The 'spinner.gif' file is the preloader image, used when an image is being loaded and is not yet ready to be displayed.
You can show or hide the preloader via the showPreloader configuration option in JuiceboxBuilder-Pro's 'Customize -> Main Image' section.
The 'spinner.gif' file can be found in the 'jbcore/classic/img/' folder (of both Juicebox-Lite and Juicebox-Pro galleries).
It sounds like you might want to upgrade your gallery to the latest version (v1.4.3) and re-upload the complete 'jbcore' folder to your web server to be sure that all core Juicebox files are present and correct.
Instructions for downloading the latest version and upgrading existing galleries can be found on the Uprading Juicebox support page.