If you use one of Weebly’s photos, the attribution is added automatically.
If you use your own or a friend’s photos and want to add an attribution, you should be able to do so by using the Caption option. This box appears when you click on a photo that you’ve added to the page already. To find the Caption option, you have to scroll down a bit.
The Caption box appears. Type the caption and credit you want:
When you move off the picture, your caption should appear below the picture.
However, if you want to use an ALT tag for the attribution (in other words, the attribution appears only if you hold your cursor over the image), you’re out of luck right now. There is an ALT option under Advanced in the same dialog box as shown above, but the ALT text doesn’t show up. Maybe Weebly will fix this eventually.