Cannot delete photos from previous iPhones on iPhone 14 Plus

First, how much free storage do you have on your iPhone? At Settings>General>iPhone Storage, my phone says 50.5_GB of 128_GB used. What does yours say? If you have less than 10% free, then you will have trouble manipulating files-- even problems deleting them. So it's important to watch the storage as you work on your files.

As Matti Haveri said, since these troublesome pictures were transferred from a computer, they are considered temporary, and you have limited ability to handle them. One thing you can try on your phone is to select them and uses "Share>Save_to_Files" to transfer them to the Files app. There you can select them and choose "Share>Save_Image" to copy them back to Photos as fully integrated images.

After doing that, you will then have three copies of each of those pictures-- the non-functional ones in Photos, the newly imported ones in Photos, and superfluous copies in Files. This might take a lot of storage space.

One way to get rid of these troublesome untrashable pictures in Photos is to momentarily turn on iCloud Photos. You get a scary warning that these will be deleted. When you say OK, then they're trashed, and then Photos will start to transfer the other pictures to iCloud. But you can turn off iCloud and stop the sync before it happens.

I did not quite follow your workflow. But if you have synced images from a Mac/PC Photos.app or iTunes.app to the iPhone, then those images are "owned" by that Mac/PC Photos.app or iTunes.app. So to delete them you have to re-sync from another Photos library or iCloud Photos (just sync an empty library if you want to delete them).