As per the documentation, the GetImageByUrl accepts these parameters:
- object imageUrl
- object (int) maxSideSize
- object (int) width
- object (int) height
- object alt
The only annoying thing is to get to the last one (alt) you have to put in the other parameters.
If you use the ASCX transformation you should be able to call the parameter by name:
<%# GetImageByUrl(Eval("IllustrationImageFromMediaLibrary"), alt: "My Alt") %>
But if you want to use the K# syntax, you'll probably need to create a custom macro to get the image and set the alt tag manually.