Hello!
Regarding the images, those don’t have that much importance than links, because when Google indexes a page, it actively searches for links and follows them. And if you have a link that it can’t follow, than it’s bad. But images are just elements of a page, Google indexes it, but there is nothing that can be followed like links, so there is no damage if you rename those.
From an other standpoint, if you rename a link, it makes Google unable to index a page completely. If you delete/rename an image, there is no problem, Google still can reach and index the page, and eventually it will just update it’s records with the changed image details.
Hope my description makes sense 🙂
On the technical side, you cannot rename images from WordPress, so the best way is to upload them again, but I am not sure if that is a good idea, because than you will have to include all of them in your pages/posts, and based on the number of images you have, that can be a pretty tremendous job. Instead, if you are concerned with SEO, it is most important to use Alt information or Alternative Text. You should always fill that in when you are using images, because Google ranks that information pretty high.
Regarding tags, that is a tricky thing. If you rename a tag, through Posts->Tags in your Dashboard, WordPress will rename the tag everywhere you have used it.
As far as I know, you cannot add tags to your images, and regarding the portfolio items, it depends on how those were created. Meaning, portfolio items are not available out-of-the-box in WordPress, they are so called custom post types, which need to be created with some php code. So it depends how it was set up back in the day, if you see the tags meta box in your portfolio editing page (like for your posts), you can add and use them just like for posts, if there is no such thing, than it can’t be done.
Having said that, many people consider using tags for SEO a bad thing, because it creates duplicate content. Actually, Ross Jone, a former Google employee has a really interesting talk about this on WordPress.tv, you should definitely check that out. If I remember right, he mentions a website where the owners used a couple thousand tags for about 400 posts, so they had about 400*1000=4000 pages which contained duplicate content. (If this is not the video it is mentioned, just let me know and I will check where it is, because it really relevant in this topic).
Cheers,
Toth Balint BT
