When adding images to a RoboHelp project, it's a best practice to add Alternate Text and an Image title to every image.
Alternate Text serves two purposes. The alternate text appears in the published output if the image cannot be displayed to the user for any reason. For visually impaired users, assistive devices such as screen readers read the Alternate Text out loud, making alternate text critical if you're trying to meet or exceed Section 508 standards.
An Image title is a brief description of the image. This title appears when the user runs their mouse over the image.
If you plan to reuse an image in multiple project topics, I've created a short video demonstrating how to repurpose the image, the image title, and its alternate text all in one shot. (Spoiler alert: the copy and paste command is your best friend!)