Put your pictures in an online service like PhotoBucket.
If you use Photobucket, do this. Once they are there, hover your mouse over the one you want to place here and you will get a small box with some choices like Email & IM, Image Code, and Image Thumb. Put your cursor on the word Image Code and left click. That will copy the image code.
Then come back here to the message editor. Place your cursor where you want the pic, right click and select Paste.
What you should get is text that reads: [img]full url of image location and file name[/img]
Here is another way to do the same thing. If you hover your cursor over the pic of my truck and trailer in my signature and then right click, one of the choices you will get is Copy Image Location. You can do this with any pic on the net. Then you just have to paste it inbetween [img][/img] and you are all set. Kind of like this:
Alternately, you can just get the full url of the location and file name and then paste that into the little window you get in the message editor when you select the little postcard icon.
Pics in forums are best when they are no larger than 640x480. Services like Photobucket let you edit your photos, and one of the edit choices is to resize the pic. Here is a good example of a properly sized pic: