Here's my guide for Windows 7 / 8 users.
I'm assuming that you can get your pics from your camera or phone onto your PC and know where they are.

In Windows, use explorer to find the pics. It's easier if you show the files as large icons.  The leftmost of three icons in the top  right lets you choose. Find the picture you want and hover the mouse pointer over it.  You should see a box with the file size. If it is over 250kb then you need to resize it as follows:

Right click on the picture and select 'Open With' and 'Paint'. It should appear in the paint program. Use the zoom slider at bottom right until you can see the whole picture. If you want to 'crop' unwanted parts of the picture then make sure the 'Home' menu tab is selected and click 'Select' and then 'Rectangular Selection' in the menu bar. Position the new cross cursor at the top left of the wanted content, hold the left button and drag down to the bottom right of the desired area and release the button. Click 'Crop' on the menu bar and the picture should now just contain what you want. Click the save icon, a floppy disk icon at top left.
Now click the first menu tab (to the left of Home) and select 'Properties'. 'Size on Disk' needs to be less than 250kb. If it is too big then click 'Resize'  on the Home menu tab. Select 'pixels' and make sure the 'Maintain Aspect Ratio' box is clicked. Make a wild guess about how much to reduce the 'Horizontal' number by, bearing in mind that halving the number will reduce the picture size to a quarter. Type in a new number and OK. Now click the  Save button and look at the Properties again from the left menu tab. When you are happy the pic in under 250kb  then you can close Paint.
Back on the forum, write your post and click the Browse button. Go find your pic and select it. To post more than one pic you need to press the little down arrow above Browse and a further Browse button will appear.
Happy Posting.

If anyone has suggestions then perhaps hack this post about and repost it all for clarity and so we can end up with a single guide.  :)

..or use Irfanview (excellent and free) instead of Paint. Open the picture with Irfanview, click control-R to re-size, type 640 (it's a good size) and ok, then control-S to save, rename if you want to a different file.

If you want to select a section click and drag a frame around the bit you want and control-Y.

Thanks Nigel for the link to this app.