Adding Alt Tags to GetImage("NewsTeaser")

What about using one of the overloads with the alternate text parameter as shown in the documentation? You would need to specify the dimensions though...

<%# GetImage("NewsTeaser", 200, 200, 100, "image alternate text") %>

<%# IfEmpty(Eval("NewsTeaser"), GetImageByUrl("/media/Images/Home2018/NewsDefault.jpg"), GetImage("NewsTeaser","AltImage")) %> <%# Eval("NewsTitle") %> <%# Eval("NewsSummary") %> I tried adding this, and it doesn't seem to work. The AltImage is a new field that I created, which does show up on the news thumbnail upload, but doesnt show up when you look at the source code.

The parameter for alternate image is the 5th one, as mentioned, you need to define the dimension parameters too:
<%# GetImage("NewsTeaser", 200, 200, 100, "image alternate text") %>
And if you want to use some field, you may need to use Eval< string >("FieldName")

As in:

<%# IfEmpty(Eval("NewsTeaser"), GetImageByUrl("/media/Images/Home2018/NewsDefault.jpg"), GetImage("NewsTeaser") Eval("AltImage")) %> <%# Eval("NewsTitle") %> <%# Eval("NewsSummary") %>

Does it go in the main parenthesis as above or a separate parenthesis, after the ("NewsTeaser"))?

The GetImage method, if you want to use the alt text property has 5 parameters, so you need to define them all, separated by colon:

<%# GetImage("NewsTeaser", null, null, null, < AltTextField >) %> or,

<%# GetImage("NewsTeaser", null, null, null, Eval< string >("AltTextField")) %>