Example of dynamic population for image choices options

gform_pre_render_custom_image_choices_options.php

<?php

add_filter( 'gform_pre_render', 'custom_image_choices_options' );
add_filter( 'gform_pre_validation', 'custom_image_choices_options' );
add_filter( 'gform_pre_submission_filter', 'custom_image_choices_options' );
add_filter( 'gform_admin_pre_render', 'custom_image_choices_options' );

function custom_image_choices_options( $form ) {

	if ( $form['id'] != 9999 ) {// set to the form id you want to run this on
		return $form;
	}
	
	$posts_query = new WP_Query(array(
		'post_type' => 'custom_post_type',
		'tax_query' => array(
			array(
				'taxonomy' => 'custom_taxonomy',
				'field' => 'slug',
				'terms' => 'lorem-ipsum'
			)
		),
		'posts_per_page' => -1,
		'fields' => 'ids'
	));
	
	$items = array();
	
	if ( !empty($posts_query->posts) ) {
	
		foreach( $posts_query->posts as $post_id ) {
			$items[] = array(
				'value' => get_the_title($post_id),// or set to whatever you want the "value" to be (eg $post_id)
				'text' => get_the_title($post_id),
				'imageChoices_image' => get_the_post_thumbnail_url($post_id, 'medium'),//set to the image URL of the size you want to use 
				'imageChoices_imageID' => get_post_thumbnail_id($post_id)// set to the image ID
			);
		}
		
		foreach ( $form['fields'] as &$field ) {
			if ( $field->id == 9999 ) {// set to the relevant field id
				$field->imageChoices_enableImages = true;
				$field->choices = $items;
			}
		}
		
	}
	
	return $form;

}