I am new to blogging and learning as I go. I wrote a few posts with lots of images and now that I am learning about SEO, I realize I need to add alt text and have names that are relevant to the image (versus IMG_6473, etc). I already changed my permalinks about a month in and understand how that can create a mess. The last thing I want to do is ruin my site by "redirecting" every image link. Not sure if I should just move on and fix them as I post or go back (I have 1700 images in my media library - all missing alt text, all missing image names that mean anything). Such a mess, but I am prepared to take it on if it is safe for my site and will improve my SEO. I have a travel blog and these posts are some of my best National Parks so I am hoping to eventually have them rank. Thank you for any help.
I actually looked into this recently, but I haven't implemented it yet. So this is a theoretically solution.
First of all... Keep in mind, that this process pretty tedious and annoying. The core reason to this is that WordPress stores everything as a post (also images). So 'just having the correct images in the uploads-folder' doesn't quite cute it. You also have to have WordPress generate the so-called 'meta-data' for each image. Otherwise you can experience all kinds of wierd results.
Here's how I would do it:
Step 1: Prepare for quite the operation
Don't do this on a Friday afternoon. I'm betting that what you're looking at is quite the operation.
Step 2: Update WordPress + Backup
How images are generated and stored hasn't changed much, but still - the journey you're going on would require the sharpest tools you can get.
And I would download all images and take a backup of your solution (perhaps using the Updraft plugin).
If it was me, then I would also make a backup with the plugin Duplicator (perhaps the premium version), so I can slap the old working website down on top of everything and roll back that way, if things goes sideways.
Step 3: Rename files
As said earlier, I haven't used it yet. But I would use this plugin here: Media File Renamer. I've used other plugins Meow Apps have made and had good experiences with it.
Step 4: Make redirects
This is where it gets dicey. Since you have 1700 images, then you would need one redirect for each image-size of each image!! Which means that if you have 4 image-sizes, then you would need 6800 redirects. Which is crazy!? I know... I know... But that this is why I haven't done it yet. I can see that the author if the Media File Renamer-plugin has written a bit about making an integration with the famous Redirection-plugin, but it seems like that nothing came out of that.
Step 5: Check it
When I've done operations like this previously, I usually find some corner-cases that doesn't work.
So I would also install the Redirection-plugin and then keep an eye on all 404-errors in the days after you've renamed everything.
And then click around, seeing if any images gives you a 404 (and perhaps checking in the console, to get the accurate path that gives that 404.
I also considered this for SEO-purposes. Because there isn't a SEO-article out there that doesn't mention this. However... The question is, what is better for SEO:
A) If you spend a day (or two days) of work, renaming all images and inserting alt-tags on them all, also creating a monster of redirects to push in front of you for the rest of your life.
B) Accept your old lazy naming-strategies. Maybe just reupload the images on the most important pages or the most important images, naming them properly and insterting alt-tags on them. And spending that extra free day of your life making new good content on your blog.
It always depends... But if you can go with B - then I would do so.
A romantic twist
Also remember, that there are sooooo many websites out there with image names like: IMG_6473.jpg. So Google and other search engines are probably trying their hardest, trying to interpret those images, figuring out what is on them, without relying on 5-10 words from a title or and alt-tag.
You know what they say: Images doesn't just say however many words that can be fitted in the alt-tag.
So while all SEO-consultants are trying to rename images and insert alt-tags, the geniouses at Google are trying to make it redundant to do so. My point isn't that it doesn't matter. It's only supposed to make you feel better, in case that you choose option B from above. :-)