You can not do it automatically, it will not work. You have seen th requirements before you can upload avatar on this forum and anything larger than 120px wide, 80px high, and 100 KiB size images will not be successfully uploaded but bring up error.

There are many apps that are available that you can use to resize the images. These kind of applications also will reduce the kilobytes of the image to less than 100 kilobytes.