You can use your phone/tablet. Upload the image to i.e your google photos or disk. On your phone/tablet dowload this and from the Seestar app go to "Community" pres the "+" and select "Image", pres "+" and "Upload Image" then you have access to your phone/tablet internal album.
You can also connect a usb cable to the Seestar or connect via WiFi to transfere the image to the device. Then you should be able to find it in the app under "My Album".
I guess its also possible to post them directly via your browser but i dont know the url.