|Log in, go to "My Files" and upload it there.
Then when you next add or edit the listings for your events the image will be available for use in the form. (and also the wysiwyg editor if you are using that)
The file formats for images should be gif, jpg or png. (those are the formats most browsers can display)
The filenames must end in .gif, .jpg or .png
Make sure you use sensible filenames.
Do not use spaces or punctuation in filenames (apart from the required fullstop just before the file extension)