How do I resize/change resolution of pictures easily?

I'm not aware of any "native" or "pre-installed" capabilities. But the easiest way, if you need it more than once a year, is to use a suitable app. You can find some candidates in my app-listing Other photo editors. Examples:

• JustResizeIt! allows to resize single images or entire batches. Comes for free, size less than 1MB, decent permissions (access storage and internet only).


• Image Shrink Lite is even smaller (less than 400k), and additionally allows you to remove Exif data (location etc.) No network permissions requested, just read/write storage. Free again.

Both mentioned apps also have paid counterparts; I didn't check the differences between those versions. For more candidates, see the list linked above :)