The short answer: You don't.
When you see an image "posted" here, what's really happening is the image is already "hosted" on a web site somewhere else, and the message board combines the text from The Survival Forum with the image from Some Other Web Sites.
For example, if you look at this page:
http://www.focazio.com/equipped/omp.htmlYou'll see a picture of a flashlight on that page, which is made up of words and several images. Each image actually has it's own web address.
The web addresss of JUST THE PICTURE of the flashlight is:
http://www.focazio.com/equipped/images/princetone-tec-light.jpgLike every image on that page, I can tell ANOTHER web site to show the image in another context.
So, for this forum, if I want to show the flashlight in my message, I click on "Image" (just below where you type your words) and then put the web address of the picture I want to show in the box that pops up.
But the key concept is that the image is already online somewhere, and you tell this forum to go get it and show it.
Now, as far as image SIZE and all that...well that's an issue that is NOT simple, and can't be MADE simple. Eventually, you'll have to learn to deal with things like "Pixels" and "Scaling Images" if you want to use digital photography at all.
The thing that confuses novice users of digital images is the fact that what you see on your screen - say an image that appears to be 3"x5" on your screen - has nothing to do with the size of the file that makes up the image - or even the size of the print you can make with the file. You can make poor quality, blocky 8x10 print from a file that's 640 pixels by 480 pixels (about a "megapixel") or you can make an amazing quality 8x10 print from a 6 megapixel file.
It's a bit of a mental jugging act - you have to think about the end use of the image, and adjust accordingly. For example, an image to be emailed or posted online does NOT have to have much information - computer screens can't show as much information as print - so you're not going to send the original file your nice new digital camera took - it would be much too big (as you learned). So you have to "scale" the picture down to something that's better for screen display.
It sounds complicated becasue it is. Hopefully you can find some online tutorials to help you out further.