To attach a file, select the "Edit page attachments" option at the bottom of a page. (If this option doesn't appear, you probably aren't logged in.) From the attachments page, you may add a new attachment or edit existing ones. Once attached, an attachment cannot be deleted from that page or renamed - so make sure the attachment carries the proper name on it (and proper extension) before uploading.
When editing an existing attachment, you may upload a new file to replace the current one in the attachment and/or enter a new description for the attachment. (If you enter a description and select "update" without selecting a file to upload, the attachment description is updated without changing the file.) Old versions of an attachment are saved.
Attachments do not need to be attached to the same page that they are called from. If the same attachment/image needs to be used on multiple pages, simply refer to the first page which has the attachment attached to it or create a separate page for storing attachments, which acts as a folder for storing files, in essence.
To refer to an attachment from within a wiki page, use [[PageName$AttachmentName]]. This syntax is identical to the usual link syntax, with a "$" and the name of the attachment following the page. Image attachments will automatically be displayed inline to the page.