Find the Twitter application, and tap on it once to open the application
The Twitter application has a blue background with a white bird on it, with the word "Twitter" underneath it. Your Twitter application may be in a different position in this folder, therefore you will need to find it.
Tap on the box next to "Image previews" to enable or disable the option
We can see that the option is activated when the box is blue with a white check mark inside of it. If we don't need to see image previews, tap the box to remove the check mark and deactivate the option.