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Don't give up mate, you have to be in HTML mode (top right corner button, swithc from editor to html) then put your picture link between these two commands. Hope you can get it centered!

center.png

I'll try again bit I mostly work on iPad.
Does that make a difference?

I am not really sure, never used Steemit on a iPap before. Remember to use Markdown/Html you have to select that style of editing first, its in the top right corner of your post when you start writing.

That dude I'll work it out.