I was looking for a desktop blogging client for an aspiring photo blogger. The one that seemed to meet all the requirements (free, works on a Mac, easy to upload photos stored in iPhoto, support for WordPress) is ScribeFire. It’s a plugin for Firefox and seems to be highly regarded by many bloggers. Uploading photos is as simple as clicking the “Add an image” button on the toolbar, and it will upload via FTP or WordPress’s blogging API (XML-RPC, I guess). When it opens the browse dialog for uploading photos, it selects the “Photo” media folder by default, which (as Mac users would already know) contains all your photos from iPhoto and Photo Booth.
ScribeFire also adds an item to Firefox’s right-click pop-up menu, which lets you easily create blog entries about web pages (and makes re-blogging a cinch).
The one downside compared to other desktop blogging tools such as Ecto is that you can’t do true image resizing from within ScribeFire itself. If you’re doing photo blogging, though, presumably you’ll be using some other image editing tool to get the photo looking just right before you publish it anyway, so I don’t see this as a major drawback.