This suggestion might be a bit advanced if you don’t have any kind of code / mMysql tables knowledge, so please be advised that you must know what you are doing before you do it, ok?
If your posts have some kind of text (or code) in common amongst them all (they have to, if you say that all posts have an image, right?), you might want to *serach* for a bit of text/code on every single post and replace it with an added bit of text / code where you will have paragraph HTML tag (<p>This is some text in a paragraph.</p>) on it.
Note, you *would not* delete the original text / code you already have on every post. You will grab it and add a bit of code (the paragraph HTML tag) so you will get the desired paragraph on all posts.
Those two plugins might be of your interest for this kind of task:
– Word Replacer https://pt.www.ads-software.com/plugins/word-replacer/
– Search & Replace https://pt.www.ads-software.com/plugins/search-and-replace/
Please, take care when dealing with database data. ??