How can I set an image title and description when inserting an image into a Google slide using Google Apps Script?

Although I'm not sure whether this workaround is useful for your situation, how about this answer? I always use for adding the title and description using Slides API, because I couldn't find the methods in SlidesApp. The sample script is as follows. In this case, I used batchUpdate of Slides API.

Sample script :

var id = currentPage.insertImage(imageUrl, left, top, width, height).getObjectId();
SlidesApp.getActivePresentation().saveAndClose(); // Important

var resource = {"requests": [
  {"updatePageElementAltText": {
    "objectId": id,
    "description": "sampleDescription",
    "title": "sampleTitle"
  }
}]};
Slides.Presentations.batchUpdate(resource, presentationId);
var fields = {fields: "pageElements(description,objectId,title)"};
var ele = Slides.Presentations.Pages.get(presentationId, currentPage.getObjectId(), fields).pageElements;
ele.forEach(function(e){
  if (e.objectId == id) {
    Logger.log(e)
  }
});

Note :

• When you use this script
  • Please enable Slides API at advanced Google services and API console.
  • Please prepare presentationId and currentPage.
  • Please use saveAndClose() after the image is inserted.

Reference :

• presentations.batchUpdate

Updated at November 20, 2018:

The Slides service was updated at Google's update at November 14, 2018, and several methods were added for achieving this issue.

new methods let you add alt titles and alt descriptions to page elements. The following methods have been added to the Group, Image, Line, PageElement, Shape, SheetsChart, Table, Video, and WordArt classes

• setDescription(description)
• setTitle(title)