The hint that I really appreciated (not sure where I heard it) is to ....
Find the picture you want to post on the web somewhere (anywhere) so that the picture is displayed on your browser. The important point that Groo was mentioning is that if its your personal picture, then you'll have to find a way to get it posted up on the web. Others have covered that real well.
Now move your cursor over the picture and right-click on the picture (press the mouse button on the right side - most people usually use the left mouse button to do their clicking, but this time you should use the button on the right side). In Microsoft's browser you'll see a dialog box pop up. Click (left click) on the Properties command at the bottom of the dialog box and you'll get a Properties dialog box. Notice that the third line in that dialog box as something called Address that looks like a web site address (a URL). That is the text that you want to enter into your forum message that will be converted into a picture.
Highlight that address starting from the right side of the text making sure to include the whole address including the http part and the .jpg (or whatever format it is) part. With the address highlighted hold the Ctrl key down and then press the C key. This (Ctrl + C) copies the address for later pasting.
In some forums you can just paste (Ctrl + V) the address into the text of your message, and once posted it will display the image. I think that is the way the ETS forum works.
In other forums you will need to find an "Image" or "Picture" command (usually near the text entry box - in the ETS forum it is in a group labeled "Instant UBB Code" below and to the right of the text entry box. Click on Image or Picture command. This either gives you a dialog box to paste the address, or inserts some code into the message into which you are supposed to paste the address.
If available, use the preview feature to see if the image is displaying like you think it should.
This sounds confusing, but it will quickly become second nature to you after a few times.
Give it a try. Try right clicking on Groo's aquatainer picture, copy the address where the picture is posted on the web, and then paste the address into a new ETS forum message to see if you can repost inside your message. We're a patient lot ... usually.