You can use several image services, including nostr.build for free.

If you are using Damus, you already are.

On web, you can use what the client offers, or you are welcome to use mine: https://picsur.thesamecat.io

Purchasing an account allows you to manage your uploads, and gives you some higher upload limits, that is all. It's just a way to support a crew that worked hard to make something.

I also signed up today, although it will make very little difference in my flow on Nostr, since I run my own service and also have a private bucket somewhere.

Don't let it all dissuade you! Post your content and people will enjoy it! There is nothing obligatory to content on Nostr that isn't available for free. Some of us have a bit extra, and we want it to go towards both the work and expenses of builders - of whom I can name many - who have worked for nothing but good will and a desire to see this succeed.