switched from query_posts to WP_query, not working now?

I’m not familiar with MultiPostThumbnails, but I suspect your problem is:

$flexID = $my_posts->ID;

That code appears to be trying to get the ID of the WP_Query object itself. I don’t think that exists. Even if it does, it won’t do you much good.

Rather, use this:

$flexID = get_the_ID();

That code gets the ID of the current post in the loop.

In general, make sure to review the WP_Query codex page. You can tighten things up quite a bit. For instance, your whole query can be on one line:

$my_posts = new WP_Query( 'post_type=slider-image' );