We never have to worry about using an outside site to post pictures. Our hosting site provides us with it. It's servimg.
First click the icon in the black outlined square. Second resize the picture (blue outline). 640 is just fine to use, none higher then that. Then browse, find your picture, open and then click host it. When it appears, where it says imagine, click onto the link, copy, a pop up will appear, click allow. Then paste it in the box. Easy as pie!!
Any questions please feel free to ask.