When you join a challenge and start litter picking, your photos are added to a temporary collection on your phone's storage.
When you are done and upload your session when home, you transfer the collection from your phone to your account hosted on our servers. By doing that, your litter is counted as it's passed through from temporary storage to hosted storage.
The uploading process also clears out the photos from your temporary storage once it's transferred so it doesn't take up any more room on your phone than necessary.
So if you don't upload, your litter isn't counted.
Please note: To avoid losing your data, do not erase the app or update it before uploading is complete.