That will work, however that is hot-linking to another person's image hosting account. It is best to upload the image to your own image host first and then use that URL. A google search will show numerous free image hosting accounts. For the free ones I favor Photo bucket and image shack.
Although many people do not mind if you hot link, some people are on accounts with very small band width and if too many people hot link their band width will be used up for the month and all of the pictures the person has hosted will turn into red X's until the new month starts for them.
I use a paid account because it gives me unlimited bandwidth each month, so I don't get bothered if people hot link to mine.