I am wondering in what way should I spin-up windows VM for Azure from generalized image, in order to add custom settings (from what I know it's impossible to customise the VM which was deployed from Azure market place)
That would be great if you could share a kb/guide which is describing PowerShell actions because AzureRM doesn't support GUI for now, and I am not really strong in the shelling.
I second Wayne's answer. Azure cloud market provides you with functionally to create most common VMs but with limited VMs' customisation functions.
To deploy generalized VM, all that you need is basically that VMDK in VMWare or a VHD(X) for Hyper-V which will be baseline image for your deployment (depending which hypervisor we are talking about). Follow the guide that might help you accomplish the task: https://www.starwindsoftware.com/blog/deploy-an-azure-vm-from-a-generalized-image-in-azure-rm-portal"
Deploy Azure VM from custom image: After preparing Image, you can upload on-prem image and deploy VMs in Azure. More about how to deploy VMs from upload generalized image you can refer to this official document: Upload a generalized VHD and use it to create new VMs in Azure (Notice: If you want to deploy Azure VM from generalized image, you must prepare the image correctly.)
Also, You can create multiple VMs from a generalized VM image in Azure. You can capture VM to image and use the generalized image easily.
These guides are using PowerShell.
There is another way to create vm from a generalized image in Azure RM portal which using template . Get templates from here: https://github.com/Azure/azure-quickstart-templates/tree/master/101-vm-from-user-image