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Just my 2 cents, but you could use a local app to compress the images first.
Here's an example for Android:
https://play.google.com/store/apps/details?id=com.saawanapps.photocompress&hl=en
It may take some time to do, but if it takes a long while to upload full resolution in the field, it may be worth it. Basically, the longer it takes to upload normally, the more useful using a local method would be.
I am not sure about any iOS apps that would do the same thing.
https://play.google.com/store/apps/details?id=com.saawanapps.photocompress&hl=en
You have to take the picture with the app.
You could use a photo compressor. This one worked well for me, but there are plenty of options.
https://play.google.com/store/apps/details?id=com.saawanapps.photocompress