Guessing you've a microsoft windows computer
Type in operating system in computer search bar, find out if you have a 32bit or 64bit computer.
Download microsoft sharepoint designer 2010 not the newer version as they don't contain
microsoft picture manager after that.
When you get that, you can right click on your picture and select "edit" in the bar that pops up.
Select resize to "large document" hit ok, and drag your new postable picture where you want...
back in the folder or on your desktop.
Before this, just once...
You might have to set microsoft picture manager to "default" by floating cursor on your picture, right clicking and select "open with" ...microsoft pic manager.