Hello BigGuy1949.
Thank you for using Apple Support Communities. It sounds like you'd like to remove photos and videos that are within an iMessage conversations on your Mac. If so, allow us to assist in any way we can. You'll want to open the Messages app, go to the specific conversation you wish to remove photos from. Then select "Details" on the top right corner of the conversation. This will populate photos at the bottom of the "Details" menu. You can select multiple photos by holding down the Command key on your keyboard. If you wish to select all of them, click on the first photo then press Command + A on your keyboard. This will select all the photos and allow you to delete all of them at once. For more information, check out the resource below:
Use Messages with your Mac - Apple Support
We hope this information is useful. Best of luck.