Speaking from theme designer and blogger standpoint, sometimes you will not want your design to be limited/restricted by the low resolution images you found online. Getting the high resolution (usually the vector format) often gives you the flexibility (colors, dimensions) to match your designs.
Here’s few logos, including WordPress, Blogger, Digg, Del.icio.us, etc I have in my storage that I’d use whenever I need to hook them up with any part of my blog-related design work. Thought it will come in handy on any occasion. Feel free to download. All after jump.