It's now stuck in your Outbox.  Try disabling the Preview Pane
(View/Layout), then change the folder in your Folder List to the Outbox and
find the stuck message by highlighting it, then delete it.

If you still cannot delete it, then determine the location of your message
store folder (Tools/Options/Maintenance and Store Folder button will tell
you) and close Outlook Express.  Open Windows Explorer and navigate to the
store folder and find Outbox.dbx and delete it.  When you restart Outlook
Express, you will have a new, and empty Outbox folder.

PS:  For large files such as this, the most practical expedient is to upload
all the graphic files to webspace if your ISP provides it and then send a
link to your correspondents to download or view the files.  This eliminates
the problem of huge messages clogging your correspondent's Inboxes and
allows them to review the pictures at their leisure.