I want to make space on my iMac 27 by deleting pictures from the photo app.

How have you copied the images to an external drive? Hopefully by exporting originals. (or plan export if you want to keep the edited versions but lose the originals.

Are you wanting to move all your images off the mac onto the external drive. If so, all you need to do is copy the photos library to the external drive (But make sure it is formatted Macos extended journaled)

See here.

https://support.apple.com/en-gb/HT201517

Generally, I don't recommend moving an Apple provided app off of the system volume. I just don't know if that would end well. I mentioned this because you seem to be considering it. Moving the library that stores your photos can be done with some basic requirements in place.

Since you're considering moving some photos, but not all, I would ask why and if you've already started? And, if you have already started, how did you do this?

If I was in a situation where I needed to free up space on my internal drive and moving my photo library was an option, this is how I would do it:

1. I would purchase an external SSD such as a Samsung T5 or T7 or a Sandisk Extreme Portable that was at least twice the size of my existing library and other files I was planning to move so I have space to grow.
2. I would attach the drive to my Mac and format it using Disk Utility as a Mac OS Extended or APFS drive. Mac OS Extended is older and better supported by third party recovery utilities in case something goes wrong. APFS is newer and optimized for SSDs. I use APFS because I backup everything and am not worried about ever having to recover from a drive.
3. I would set that external drive to Ignore Ownership in the drive's Get Info properties.
4. I would move the original photo library to the new drive. If I copied it, I would be sure to delete the original so I'm not wasting space.
5. I would open the library from the drive by double clicking on it. If I instead opened Photos from the dock or launcher, I would get a prompt to locate the library, in which case, I'd point to the external drive to open it.
6. If I use iCloud Photos, I would go to the Photos > Preferences menu and click the button to set the library as the System Photo Library.

Let me know the answers to the questions I posed above or if the scenario I present sounds like it will work for you.