You have a few options. When creating a post you can add an attachment by clicking the "Manage Attachments" box below the message you're typing. You can upload files from your computer to the forum, and they will be displayed as a thumbnail attachment.
Alternatively, in your profile you have the ability to upload photos to an album hosted by AVF (see right-hand column in your profile). You can then link these in your post using the BB code supplied. This provides a full-size image in your post.
Finally, you can use a third-party image hoster such as flickr, photobucket or imageshack, and again, images hosted here will result in full-size images in your post, provided you have linked them properly using the
button or the appropriate BB code.