Uploading pictures from your iPhone to Flickr couldn't be easier. Many photo hosting and printing sites now allow you to upload pictures by email. (Of coure check with your host or printing service to see if they offer this option.) For Flickr, visit flickr.com/account/uploadbyemail. They'll assign you a "secret" email address. Just mail photos from your iPhone to that address and it loads right into your Flickr account. For Costco, mail your images to firstname.lastname@example.org. You'll receive a confirmation letter at the address you used to send the message. The letter includes a link to the Costco Photo Center website where you can print your photo.
Unfortunately, the iPhone automatically scales your images when you choose the Email Photo option. It does this to save the mailbox of the person you're sending your images to. That's great when you're sharing cute pictures of the family or dog. It's not so great when you want to print out your emailed photos. To address this problem, I've written SendPics, a utility that allows you to select a photo from your iPhone and email it at full resolution. You'll find a copy of this utility at iphone.natetrue.com and via Installer.app. Your iPhone must be modded to allow third-party applications in order to take advantage of this utility