Can Xojo iOS access a random picture from the user's camera roll?

That’s pretty much the question.

I’m not sure if that is possible, or only if it’s limited to the iOSPicturePicker popping up and the user has to make the active choice of photo.

I just want to grab a random photo from the user’s cameraroll to use as a canvas backdrop…

No, and thank $DIETY for that. Two words: dirty pics.

You may be able to achieve a work-safe version of what you’d like to do though. Open up the iOSPicturePicture and ask the user to select a number of photos they’d like to display at random on this backdrop. You kill two birds with one stone, you avoid dirty pics, and you get an assortment of approved photos to display.