It is a often good idea to reduce the image file size before sending to someone via email or uploading to sites like eBay or Local Sales Network. Today's smart phone have high resolution cameras that create really large image files. Many times that high resolution is not needed or even wanted.
To the rescue come phone apps that can reduce the file size (and resolution) of image files right on your phone and them make them available to "share" in several directions.
The one that I use is called Reduce Photo Size. I find it easy to use and a handy tool to add to your Android Smart Phone.
Here is a link to its listing on Google Play Store: