Ejabberd: mod_http_upload is included in ejabberd-core from version 15.10 on
After you have connected to a server with support you should see two new buttons.
When using the button for sending images you will see an image preview in the dialog for selecting the image.
Also when the upload is completed a thumbnail will be sent to the recipients with XHTML-IM.
If the recipient doesn't support XHTML-IM they will just see the URL instead. Gajim supports HTML-IM and will always display the thumbnail.