For a custom mini-map image, check out:
http://world-editor-tutorials.thehelper.net/minimap.php
For screenshots, you take them with Print Screen. They will appear in your Warcraft 3 screenshot folder, but as TGA files. As such, you may want to use some freeware tool like
irfanview to convert the images to PNG or JPEG.
Afterward, I recommend making an album. You can go to your user profile, and then scroll down to "Add Album". Make a new album and add pictures to it. After uploading one, you can right click it and select "Copy Image URL". Then when you post your map, you simply put it under [IMG][/IMG] tags. For example [IMG]http://www.hiveworkshop.com/forums/images_all/avatars/warcraft0298.gif[/IMG] will give:
(your avatar).
Also, I recommend that you preview your post before uploading the map entirely. You can just go to any forum, click "Add Reply", and copy and paste your map's BB code. Just click "preview". Make the changes you need. Once it looks fine, just copy and paste that code into your map upload description.
Also, I've moved this to WEHZ since it isn't really a request.