This is a bit of a rambling post...
On further experimentation, you could consider using the inbuilt [gallery] shortcode to help with displaying post images:
Add to child theme functions.php
//This adds Post-Thumbnail support
if ( function_exists( 'add_theme_support' ) ) { // Added in 2.9
add_theme_support( 'post-thumbnails');
}
// This allows shortcodes to work in widgets
add_filter('widget_text', 'do_shortcode');
And in a typical text widget add:
[gallery]
Then you would upload images as normal when writing a post (so that it becomes attached as a Gallery) and it will display all the attached images in the sidebar. The default shortcode isn't very customisable so you could consider using another plugin that extends the functionality.
One issue occurs when trying to use the same image in different posts; I only found out just now that a 'feature' (or liability?) of native WP media handling is that once an image is attached to a post, it can't be attached to another post, unless you upload that same image again or detach it.
There seem to be at least two plugins built to get around this: Media Library Plus plugin and (currently-alpha-beta) File Gallery.