Adding an image onto a Captivate slide is as simple as choosing Insert > Image. Once the image has been added to the project, adding additional instances of the image to more slides is a snap. All that you need to do is go to the Library (Window menu) and drag the image onto a slide.
Once an image is imported into a Captivate project, a link is automatically established between the version of the image within Captivate and its source file. If the source file is ever edited, every instance of the image in your project will usually get the update automatically.
I've come across instances where the automatic update feature didn't work. In that case, getting the updated image is still easy enough... right-click the image on the Library and choose Update to open the Update Library Items dialog box.
Note: You can update multiple Library assets by shift-clicking or control-clicking items and then showing the Update Library Items dialog box. If you don't want to update some of the selected items, you can remove the check mark to the left of an items name.