Create Azure VM from image created under different subscription

I don't think you can use images created in one subscription to create VMs in another subscription. You would need to copy those images into your subscription. Since images are nothing but Page Blobs in your storage account, you would need to copy them into a storage account in your target subscription, create images off of them and then deploy VMs. I'm not aware of any other way around it.

Regarding your comment about requires purchase of third party software, that's not really true. You don't have to buy 3rd party software. The main thing is to move your VHDs (which are page blobs) from one subscription to another. Do take a look at Step 1 - Copy Blobs in that blog post. It has a link to console application with source code that you can use to copy blob across. I used Cloud Storage Studio to explore my blob storage. You can use any other storage explorer to check the contents of blob storage (including Windows Azure portal). Cerebrata recently released a free blob storage explorer which you may want to check out: http://www.cerebrata.com/labs/azure-explorer.

Unfortunately that's the only way that I know of to create Azure VMs from images created under different subscriptions.