Nevo free version theme mainly uses default size inside media settings. Settings → Media
If you are using the Nevo theme for the first time I recommend you go to the media settings and determine the image size you use.
It is very important in reducing image size which reduces load, responsive images and faster website.
Image sizes: used in archive page, single page, sidebar…etc.
For sites that are already active
You can keep the settings as is or create new settings. But, older images you previously uploaded are unaffected, it can create non-uniform dimensions.
I usually use the Regenerate Thumbnails plugin to regenerate new image sizes. Please consider this as recreating the image may take some time depending on the number of articles and images used.
Settings Regenerate Thumbnails plugin. Plugins → Add new.
Enter the keyword Regenerate Thumbnails in the search box and install and activate it.
Then, go to Tools > Regenerate Thumbnails, WordPress image-related settings being found here.
By default, the Skip regenerating existing correctly-sized thumbnails option is checked. Uncheck this box to use the Regenerate Thumbnails plugin to adjust all your images.
But, leave it checked if you want to save your server’s resources.
This is recommended if you already updated some of your image sizes such as through your theme. You can also choose to check the box underneath this option to Delete thumbnail files for old unregistered sizes. Selecting it frees up space on your server.
Make sure you don’t need these images before you choose this setting. If some of the images are in use on your site and you select this option, they’ll be deleted and will no longer be displayed on your site. Visitors will see a broken image.
You may also see the Update the content of posts to use the new sizes checkbox, which you can choose if you want the images that you’re currently using in your posts updated.
After you chose the options you want, click the button to Regenerate Thumbnails, medium, large, and new file sizes included.
Then, wait for the process to complete. Depending on how many images you have on your site, this could take a while so grab a coffee or glass of water while you wait. It typically only takes a few minutes for larger websites on an high-performance server.
You’ll see the “All done!” message when the process is complete.
Your thumbnails and images should now be regenerated.