Choice 1: you set lower resolution of your camera to retake the photo and then upload!
Choice 2: Download photo editor apps from apple or google play store: such as SNAPSEED to open your big size photo and resize the photo saving as a smaller file to upload!
I use SAMSUNG phone already built in free photo editor to easily resize and upload here! Even the highest resolution (12,000 x 9,000) 108MP mode about 45MB a photo can be easily resized and upload conveniently with built-in apps!
I think your mobile camera shall have built-in photo editor software apps, maybe you call your mobile phone customer services to ask about it!