Because my memory's going, I often can't remember how to do things on the PC either. However, when it comes to resizing pictures I put them into Microsoft Paint, then click on the 'Resize' button at the top on the 'Home' tab, but under the 'Crop' button.
I then get a small screen with several boxes on it. This is the small screen I'm talking about.
On this screen I click on the 'Pixels' button so that I can see exactly how big the picture I'm resizing actually is.
This one is 376 pixels horizontal by 420 pixels vertical.
I can now change the numbers in one of the boxes, and because the 'Maintain aspect ratio' box is ticked, the numbers in the other box will also change so that at the new size the picture will stay looking the same.
When I'm happy that I've got the size I want, I click on the 'OK' button at the bottom of the screen and it closes. Back at the opening screen again I click on the tab to the left of the 'Home' tab in order to open the Save and Print menu.
Note that I've put a red circle round the tab I'm talking about.
With the Save and Print menu open I click on the 'Save As' option in the drop down box, and another box opens to the side of this, showing a list of formats the picture can be saved in, including PNG, JPEG, BMP, etc.
I normally select JPEG and then once I've done that I can choose what to call the picture and where to save it.
With the picture saved I can then select it for my new avatar, or whatever I wanted to resize the picture for.