Reducing the number of pixels in photos can be done using apps that are available for ios (apple) and android (most others). On my iphone I use Photo Resize. If you can, save the resized photo into a named folder and when selecting it for upload, select that folder rather than the All Photos folder.
Most phones and tablets take photos at a resolution level suitable for printing in glossy maagazines, way too many pixels for screens of phones, tablets or home computers. They just don’t need to have so many pixels in them. 800 x 600 or so is quite sufficient for most online uses and even that size nearly fills the screen of most phones. If photos have too many pixels, the program displaying them cuts them down to a lower level, but if the photo is a 4000 x 3000 pixel file, it is 25 times larger than needed whenever it is copied or moved. When phone users are limited to specific data limits on their plans these large files consume data allocations with no benefit to the user. And they take longer to load. So cutting them down in height and width measured in pixels is really worth while.
Hope this helps.
Andrew