On this site. when you're posting a message, the yellow button with a mountain in it opens a text box where you enter the URL for the picture.
I find it easier to type the image code directly rather than use the buttons.
After you have your picture on your computer, this is what to do.
For courtesy of people on dial-up connections and ease of reading, resize the picture. If you don't have a tool to do that, here's a good freebie:
Irfanview
Install it to your computer. You'll be asked about reassigning various media types to irfanview. It's probably worth it to reassign GIF and JPG/JPEGs types if you don't have another preferred image viewer. You can then open your picture in irfanview or drag and drop your picture file into the program.
In the image menu, choose Resize/Resample. A dialog box appears.

The first area on the left tells you the current image dimension in pixels. My sample image is 1260 x 980Make sure the Preserve aspect ratio box is checked. Then you can enter a width in the highlighted blue field and the height is automatically calculated to minimize distortion as the image is shrunk. Pick a width of 800 or less. Those are generally the friendliest sizes. Save the file. I recommend using Save As so you don't overwrite your nice big original file. Choose a different file name too. Other image tools work in a similar fashion.
The picture is on your hard drive which is hopefully invisible to the internet. So you have to copy that picture to some place on the internet. In my case, my ISP provides some web space along with the account. So I copy my files to that space using an old internet tool called FTP. There's too many different tools to walk you through that, but if you have web space, your ISP can help you get your files copied there. It's usually simple.
If you don't have web space, then you'll have to use the photosharing sites. I'm not current on what the good photo sites are, but a quick google turns up
Image hosting, free photo sharing & video sharing at Photobucket. If any of you know or use a similar site, please recommend them. They should be free to use, just plenty of ads are displayed to pay for the site. Be warned, these companies have a history of going under so keep a copy of any picture you post on them so you don't lose data. Each of these sites has a tool to upload your photos to them. Once your picture is on a site, it then has a URL associated with it. If it's on personal web space, it too has a URL. Bring it up in your browser and you can copy the URL right out of the browser address bar to use. Now that you know your picture's URL, you're ready to post the picture in a forum. Most forums use the same picture code but the buttons don't always behave the same.
Phil