I've got Mircrosoft Photo Editor on my laptop which isn't very good and I couldn't change anything either.
When Minim was helping us with the photo's, did you download IrfanView? It's completely free and I haven't experienced any problems with it. It's much easier to edit in this and I found using 400 as the width, as recommended by Minim, everything else adjusts automatically and is an ideal size for posting photo's. I can now work from 400 to make things larger/smaller. I've even figured out how to add text!! :rock:
I could do a guide for Irfan if you like?