I use PhotoShop Elements an old version. It’s pretty intuitive (especially for Adobe…) and there are several programs to use.
Basically you choose a resolution and a display size. The key is to use 72 lines per inch of resolution, any more is wasted transmission. The screen will not display anymore than that.
For sizing, you are going to choose a pixel size of HxW (or vice versa). I like 800×600 for most all things. Keeps the transmission size down and give more than enuf detail for 95% of the things I send out.
There are a number of programs to use, but I’ve been using Adobe for so long, I don’t keep up on other alternatives. I know some of them are freeware.