I need to convert approx 250k images from having a content type of application/octetstream to image/jpg
is there a powershell script I can utilise to do this or some sort of command code?
I don't think Windows Azure PowerShell Cmdlets has a readymade Cmdlet to do that. If you're open to use 3rd party product, you can try Cerebrata's Azure Management Cmdlets. It has a Set-BlobProperties cmdlet using which you can set the content type.
You can also do it through code as well. Essentially what you would do is fetch blobs from the blob container using ListBlobs or ListBlobsSegmented method and then iterate over these blobs and update the blob's properties via SetProperties method of CloudBlockBlob.