Check out this phpFlickr wrapper for the Flickr API.
Here is a first test of the phpFlickr library.
Here is a second test of the phpFlickr library, which requires Flickr authentication.
Here is a third test of the phpFlickr library, which looks into Eric's private Flickr images, but should not require you to authenticate to see the images.
First approximation
Next steps
Work on the uploading of images.