I assume that you mean you are using web access to Yahoo Mail.
You don't need any external program to send pictures.
Just create your e-mail then click the "attach files" button immediately beneath the subject line.
On the page that appears, click the first "Browse" button, which will open a dialog so that you can locate the picture. Browse to the folder where you've stored the picture, and double click it to attach it to the e-mail.
You can attach additional pictures (or any other files) by clicking clicking the other browse buttons.