Fist upload your pics, there are several ways. Here are just a few:
right click at the picture you want to use at steemit.
Select the "Copy Image Address" option .
The easiest way is just to click the picture button on top and past the address.
or you use the markdown code for it :) At the end it should look like this.
And what about size? How should we pick size?
???? Iy you want to change the size of a pic, you can change it at the html code of the picture width="" height=""