When new image sizes are added in the theme using Image Size option under Dashboard > JupiterX > Image Size in the Jupiter X theme, it only applies to the newly uploaded images. In that case, the old images need to be updated with the new dimensions before it’s visible on the page/posts. The thumbnails can be regenerated using the Regenerate Thumbnails plugin.
How to install a plugin, you can read in this article.
The plugin allow you to regenerate all images at once as well as individual thumbnail images.
Regenerate All Thumbnails at Once
1. From WordPress left menu, go to Tools > Regenerate Thumbnails.
2. Click on the Regenerate All Thumbnails button and wait for the process to complete.
Regenerate a Specific Image
You can regenerate specific images (rather than all images) from the Media page.
1. From WordPress left menu, go to Media > Library.
2. Click on the list icon on the Media Header filter to show the media items as a list. Hover over the thumbnail to show the Regenerate Thumbnails option. Click on it to regenerate thumbnails for that image only.