In an answer, I have a long list, and for each item there is an associated image. If every image is displayed after each item, it takes a lot of space. I would like the image to show up only when you click or mouseover on every list item. Is that possible ?
This is not possible to do with the StackOverflow Markdown editor. See the Editing Help page for a list of the most basic formatting that is available to you on StackExchange.
Additionally, for more details about what you can do in the markdown editor, see the Daring Fireball Markdown Page.
Since you have lots of images, you could try resizing them as smaller images and placing them next to your list items. With that said, don't let your question get too distracted with images, make sure your question has enough substance to be both constructive and not too localized/specific to a problem that no other person on the Internet will have. Good luck!
For a workaround, you may use my Thumbnail Uploader script with options "small" and "link to full size".
(installation instructions there)
This works like the normal image uploader, but it inserts a small version of the image and links it to full size. This way, you can use large images without worrying about the text reflowing,
Note that Imgur's resizer sometimes crops images that have a large aspect ratio. In this case, use the custom option and specify the height in pixels (leaving the width box empty locks aspect ratio). The custom option is also pretty useful if you want to have even smaller thumbnails.