Ok going to go through some steps, tell me if you still cant work it out afterwards.
First go to an online image hosting site, I personally use Imageshack, others may vary, you can ask the other forum members here if you need others 
(this is a free service btw)
If you are using imageshack, remember to login, else it will delete your pictures at some point.
and take notice of the upload button, now you need to grab your picture, so print screen and save it in a file somewhere, preferably some of the filenames that use less space, like JPG og PNG
oh, forgot the red box on this one, but when you have uploaded a picture, click the "View Full Size" blue text shown here.
When you see the image, you gotta copy the link, putting in an image to make it abit more fool proof 
Now you have your image, and your link, now you go to the forum and click the add image button. (You might recognize the picture 
)
Finally you put in the link for the picture, and set small dimensions, for this I have (mostly) used 300x200.
Hope this helps!
PS, what picture in your dropbox am I looking for?
Sincerely
~ Kongdej
