There are multiple ways to do this.
If you are uploading a file from your computer, you can go to Tinypic.com and upload. It will then give you a few codes. You want the code for Forums and Message Boards I believe. It will have [IMG] in both sides. It would look something like this: [IMG]----------[IMG] (Where - represents a name of the picture/file.)
If you are uploading a photo from Google Images, you could either save the image to you computer as a ..jpeg or .png file (.png requires less memory). When you save the file, you could then do the above method.
If you do not want to save the file, you will have to copy the image URL and paste that into Tinypic. (There will be a button to select that says "URL".
You can always just delete the photo off of your computer when it's done.
Don't forget the 300x500 Pixel limit for images. (That's pretty large anyway, so you should be ok for most things)
Hope this helps!