The reason that random numbers and letters have been added to the end of your image name is that there are 2 or more images that would have had the same name after editing with our Bulk Image Edit app.
Shopify does not allow multiple images to have the same name, so if Bulk Image Edit is used to change the file names and any of them have the same name, Shopify will add random characters to the duplicates in order to prevent having the same name.
For example, here we have a task which has renamed 1 product with 14 images without image position:
You can see from the screenshot: even though, we change the file name from Food-Pet-Mats-Blanket-for-Cats-Dogs-14.jpg (14 being the image position number) to Food-Shaped-Pet-Mats-Beds-Blankets-for-Cats-Dogs.jpg
When we check the image again, we can see that a random string has been added by Shopify: So the file name is changed to Food-Shaped-Pet-Mats-Beds-Blankets-for-Cats-Dogs_10d0a30a-e694-4a29-b8d7-fd1cb7ede3e3.jpg
It is recommended that you add a unique identifier to your image names, such as option names, or image position numbers when editing your image names.