Prior to iOS 8, deleting images in individual message threads was tricky business. There wasn't an easy way to view all of them at once which made deleting more than one at a time a pretty tough task. Luckily, the details view in iOS 8 changes all of that and makes it super simple to delete lots of images and attachments at once, an incredibly useful trick if you need to free up some storage space in a hurry!
How to delete multiple images at once in the iOS 8 Messages app
Launch the Messages app on your iPhone or iPad running iOS 8 or higher.
Tap on the thread you'd like to clear images from in order to open it.
Tap on Details at the top right.
Press and hold on one of the images you'd like to delete under the Attachments section.
Tap on More in the popup menu.
Select all the images and attachments you'd like to delete.
Tap on the Delete button in the bottom right when you've got them all selected.
Tap on Delete Messages in the popup menu to confirm the delete.
That's all there is to it! The images you chose to delete will no longer be saved in the Messages app, eating up space. You may have noticed that when you highlight multiple images, you also get an option to the left of the trash can to save them. This is a great way to save multiple images at once in a messages thread, so keep that in mind as well!
Now give it a try and let me know how it works for you!