Just create your own view for ImageFile, add an Alt-text-property to your Image class, and display the alt text in your view. You can have a look at my blog post that touches on this topic: https://blog.novacare.no/culture-specific-image-properties-in-episerver/

Or simply refer to a default Alloy site, there you will find the view /Views/ImageFile/Index.cshtml :

@model ImageViewModel

<img src="@Model.Url" alt="@Model.Name" title="@Model.Copyright" class="image-file" />

Just add the alt-text-property and replace @Model.Copyright with @Model.AltText and your good to go!

Hey Tomas, thanks for the quick answer. We are able to use this successfully for images rendered via `@Html.PropertyFor()` but this doesn't work for us when it's inside of an XhtmlString.

I've created a project to demostrate the problem better. In this example project when I use the rich text editor, I can add images two different ways. Using the 'Image' button lets me enter the alt text that will be specifically for the image, which isn't ideal since the alt text should come from the image itself. Or if I drag the image directly from the Media side bar, I don't get an option and the alt text becomes the file name. Below is a copy of rendered markup with this.

    <p>Image added via "Insert &gt; Image" menu, Alt text comes from this dialog.</p>
    <p>
        <img src="/globalassets/logotype.png" border="0" alt="Alt Text from insert dialogue" />
    </p>
    <hr />
    <p>Image added by dragging from media tab. Alt text is auto populated from the name property.</p>
    <p>
        <img src="/globalassets/logotype.png" border="0" alt="logotype.png" />
    </p>

This has caused trouble in two different ways. We have decorative images that shouldn't have alt text getting "Filename.png" and graphics on multiple pages that have to be individually added or maintained for each page. 

Thanks for your time!

I understand, have a look at this blog post, and the section about EditorDropBehavior.

https://world.episerver.com/Blogs/Linus-Ekstrom/Dates/2013/12/Customizing-the-look-and-behavior-in-the-UI-for-your-content-types/

You could create an UIDescriptor like this, converting all images to content blocks when dropped in TinyMCE.

[UIDescriptorRegistration]
public class ImageFileUIDescriptor : UIDescriptor<ImageFile>, IEditorDropBehavior
{
    public ImageFileUIDescriptor()
    {
        EditorDropBehaviour = EditorDropBehavior.CreateContentBlock;
    }

    public EditorDropBehavior EditorDropBehaviour { get; set; }
}

...then you should be able to control markup the same way as images rendered with @Html.PropertyFor().

Some problems:
- You will loose image preview in TinyMCE
- You should probably remove image plugin in TinyMCE to stop editors from using it

Here's my solution: https://blogs.perficient.com/2020/10/12/episerver-and-alternate-text-for-images-in-the-tinymce-rich-text-editor/ 

Basically, piggy backing off of Tomas's solution- you can create a view for XhtmlString.cshtml, and adjust the rendered results before they are sent out to the user.  You do this by looping through the XhtmlString fragments to find all that are Image based, and adjusting their resulting markup as needed.