Make sure all your images are converted to .png format, since .jpg and .gif are both proprietary format! The best way to get non-proprietary formats accepted is to use them! 'course, I doubt any heads will explode if you use .jpg or .gif or whatever.
To add icons to this page as attachments, select from the "More Actions" pull down menu (located at the top and bottom of the wiki page) the "attachments" option. From that page, simply select which icon to upload from your computer. After doing this, the uploaded icon will be available to insert in any pages of the wiki.
The moinmoin markup is the main pagetype to be used in the ubuntu wiki. For instructions as to its use, see HelpOnEditing. In order to insert icons or any other images, you simply use their wiki location like this:
This is an icon with text beside it, and I'm going to type and type and type so that you can see how it wraps, because you like to see how stuff works, since it's how people usually are able to learn, especially with code.