I have just migrated a project of mine for test cases to Microsoft's azure.
But for functionalities similar to an avatar upload I need write access to the files on the harddrive. But this is a cloud, so this is not possible. How can I build such functionalities instead? Should I use the Blob Storage or is there a better solution?
Does it make sense to store all website images (f.e. layout images) in the Blob Storage? So I would have a Cookie-free Domain for my static content?
Blob storage is definitely the place to put dynamic images like avatars. While you can write to the disk on the VM you'll be running in, you can't rely on this to be present - if your app gets moved off to another machine (which could happen for any number of reasons) this storage will be erased.
One thing you could do is store your images in blob storage, and cache them on the local VM disk (using the standard file IO mechanisms). This way you'll get pretty good performance and will save on a few storage transactions while still making sure you're not storing in volatile storage.
If you've got static images which will be completely static, these are just bundled with your application and can be referenced like a normal file. But, if you will ever need to change them, you'd need to redeploy the application - so only use this technique for images which won't need to change.
Be aware there are two types of Blobs in Windows Azure: Block Blobs and Page Blobs. Block Blobs are appropriate for media file serving, whereas Page Blobs are optimized for other work patterns.
Also consider use of the Azure Content Distribution Network (CDN) for lowering latency to clients.
Azure also has streaming capabilities which work in concert with Silverlight Smooth Streaming (http://blog.smarx.com/posts/smooth-streaming-with-windows-azure-blobs-and-cdn if interested).
