Very easy...
Do you use windows?
Most of the time, you clic on image then resize or image size, then you should have the option of choosing between inch, cm or pixels.
Most of the forums will accept a 800*600 pixels.
Then get yourself an account with a website where you can store images (I use imageshack
ImageShack® - Hosting ).
Once your photo is hosted, look for the photo properties.
You should find a direct URL to the photo.
While writing your post, select insert image icon (the one that looks like a landscape on a postcard)
Copy the URL in here and that's it.
Do not hesitate if need clarification.
Harold
PS: a lot of people use 'the gimp' in is a free SW to modify photos.