How to lower iPhone image resolution and file size *after* taking photo?

I access my Gmail email using the browser only. It used to let me upload iPhone photos in different size (original, Large, Medium, etc.). That option is now gone and I can't find methods native to the iPhone for shrinking the image resolution. I have to go home and use ImageMagick, which is very limiting. One trick is to rotate the phone while viewing the image, which shrinks the image to fit in the screen. A portrait image is shrunk to fit in the screen while the phone is in landscape orientation, and vice-versa. I can then take a screen shot and crop away the margin, but it is *very* crude way to control resolution and the image is still several fold larger than what I can achieve with ImageMagick. Using this trick twice in succession blurs the image too much. ***I find it hard to comprehend how downsampling is not available natively. Hopefully, I'm wrong?*** I would prefer not to install a 3rd party app. **ANNEX: The process I go through when I get to my computer** Using the suggestions [here](https://www.smashingmagazine.com/2015/06/efficient-image-resizing-with-imagemagick) , I do the following in Bash (Cygwin): # Bash function definition (1 line) #---------------------------------- unset -f smartresize smartresize() { mogrify -path $3 -filter Triangle -define filter:support=2 -thumbnail $2 -unsharp 0.25x0.08+8.3+0.045 -dither None -posterize 136 -quality 82 -define jpeg:fancy-upsampling=off -define png:compression-filter=5 -define png:compression-level=9 -define png:compression-strategy=1 -define png:exclude-chunk=all -interlace none -colorspace sRGB $1; } # Invocation (only 1 file at a time) #----------------------------------- smartresize inputfile.png 300 outputdir/ The "300" in the above example is apparently the pixel width. A width of 1200 is slightly worse than attaching an iPhone SE photo to gmail as Large (the sizing capability which is now gone). It is good enough for most of my uses.