Its not the size of the picture but rather the size of the file.
If you want free software that will do the job and much much more, try The Gimp. I would also recommend Faststone which is free but has a significant advantage. If you set your PC such that this prog is your preferred software to view your pics it works with almost any format including Raw for most cameras. When you double click o the file Faststone opens quickly and shows your pic on a black background with the file name and size in green in the top left corner but you can stop this showing if you wish. If you press I (that is i) it will show you the EXIF information of the picture, and if you click right on the picture towards the left side it will take you into a simple editing mode.
For attaching a file you need to go into the picture edit capability and change it to fewer colours. Reduce the number from the usual default of 16.7m to something less until the saved file size is smaller than your destination requires and then attach it.