Just my sister called me, she have a galaxy s4 , and when she want send a mail over yahoo with a attached picture , only a white screen with a loading symbol apears

Without opening mail programm

My s4 I sold in meantime , coz getting slowly when more appz installed has one time the problem

That the keyboard not pop up after press a key!

Only a factory reset helped my sister not want do this

Read on the net in meantime

A alternative firmware is avalible

That not have this slow down performance problem

I have a Samsung S3 and cannot send pictures (MMS) either.

Install an app called "Clean Master" available on the Play Store. It will clear out the thousands of junk files the phone builds up in the onboard memory, slowing it down. i bet your sister never clears out old messages or transfers photos to her memory card rather than keeping them on phone memory.

Doing a "Re-Set" accomplishes all that as well. Just make sure to make a list of the favorite apps you'll need to reinstall and save any pictures or data you don't want to loose.