I never realised people posted such big files! My usual web practice is to resize, setting the longest side to 800px and to post (or provide as link) as jpg or png.
I don’t know about the mobile devices but
Gimp would do it on most desktop/laptop platforms for those who don’t have or can’t run say a Photoshop.
ImageMagik can also be useful although I’d need to brush up on it a lot and haven’t used it in ages (and my main usage was some behind the scenes processing on a web server). For a folder of jpgs in landscape, this would convert the whole folder to 800x”height” (it preserves aspect ratio so most of mine would come out 800x530):
mogrify -resize 800X600 *.jpg.
It can be very effective but (if not as in my example, using copies in a new folder to start with) I’d strongly advise copying some pictures to a folder and testing there, before say deciding you have your answer to producing say your files as thumbnails or say reduced to post in an email, changed to a different format, etc. Otherwise, mistakes are likely to result in original images you want to keep getting overwritten...