Can I upload a picture taken from my observations and not just directly after I take the pictures?
Could you please elaborate, do you want to upload observation later and not when you make a photo? If so, you need to just take photos with your phone’s camera and not via app, and then share them with app, or you can turn off auto synchronization and observations won’t be uploaded until you choose to. You also can use website.
Are you using iNaturalist, or Seek by iNaturalist?
If the former, then @marina_gorbunova’s answer is correct. Here’s a tutorial: https://forum.inaturalist.org/t/how-to-use-the-inaturalist-mobile-app-without-an-internet-connection/11620