I want to change the inter-trial background from a gray screen to some patterned image. But I can only change the color of the gray screen.

Present the patterned image for the last stimulus. Then do not clear the screen (e.g., do not call the idle function)

The patterned image is displayed in a 25 degree window and the background is gray. Same background is used for inter trial. How can I have the patterned image displayed full screen inter trail?
Or better yet, how can I change the gray background to a different pattern both during the inter trial and during stimuli display?

I thought you just wanted to keep an image on the screen during ITI but now it sounds a little confusing. Are you asking how to stretch an image to fill the entire screen?

The ML screen is not like the Windows desktop background. There is no background layer. Just present the image and leave it on the screen during the entire trial. I do not know whether you script in v1 or v2 and whether the image is an TaskObject or an adapter, but see the attached example and the post linked below.
https://monkeylogic.nimh.nih.gov/board/read.php?2,12,12#msg-12

Sorry for the confusion but this helps!