Bad filenames which are not actually related to the image are not good for SEO. The reason is; every search engine loves more descriptive and meaningful filenames.
Another draw bag of bad filename is that they sometimes break your images in web browser.
The problem comes when you host a lot of media files on a multi-author WordPress install because there you cannot monitor everyone to upload images having good and meaningful names.
Here comes the use of a WordPress plugin called Clean Image Filenames.
The plugin works by auto-enforcing clean filenames on images and other media files you upload to WordPress media library.
Hence, it improves your website’s search engine optimization.
So How It Works?
Bad image filenames like Château de Ferrières.jpg or Smörgåsbord.png are changed to nice and clean filenames like chateau-de-ferrieres.jpg and smargasbord.png.
It simply replaces accent and special characters, like Swedish or German umlauts, in the filename uploaded to your media library.
You should know that the range of file can be extended using a filter in your theme or plugin.
How To Use It?
Install and activate Clean Image Filenames plugin in your WordPress site. Upon activation visit your admin area dashboard Media -> Add New page and upload a new image file with accent characters like Smörgåsbord.png
As soon the image is uploaded, the filename will be filtered by the plugin to smargasbord.png. That’s what all this plugin does for you.
Renaming Filename Based On Post Title:
File Renaming on upload is another good plugin which provides you with more powerful controls and options.
Unlike to Clean Image Filenames (which is activate and forget plugin), File Renaming on upload allows you select various options on how to rename an individual filename after upload.
You can auto add your sitename in image file, remove strings & accents, rename file based on post title and more.