Validation not working on image type in Sanity document

export default {
  name: 'illustration',
  title: 'Illustration',
  type: 'image',
  fields: [
    {
      name: 'alt',
      type: 'string',
      title: 'Alt',
      options: {
        isHighlighted: true, // <-- make this field easily accessible
      },
    },

  ],

};

export default {
  name: 'soloGuidePage',
  type: 'document',
  title: 'Solo Guide Page',
  icon: RiPagesLine,
  fieldsets: [
    {
      name: 'tile',
      title: 'Listing Tile Fields',
      options: {
        collapsible: true,
        collapsed: false,
      },
    },
    {
      name: 'hero',
      title: 'Hero',
      options: {
        collapsible: true,
        collapsed: false,
      },
    },
    {
      name: 'mainContent',
      title: 'Main Content',
      options: {
        collapsible: true,
        collapsed: false,
      },
    },
  ],
  fields: [
    {
      name: 'tileTitle',
      title: 'Tile Title',
      type: 'string',
      fieldset: 'tile',
      validation: (Rule) => [Rule.required().error('Field is required')],
    },
    {
      name: 'tileImage',
      title: 'Tile Image',
      type: 'illustration',
      fieldset: 'tile',
      validation: (Rule) => [Rule.required().error('Field is required')],
    },
    {
      name: 'tileText',
      title: 'Tile Text',
      type: 'text',
      fieldset: 'tile',
      validation: (Rule) => [Rule.required().error('Field is required')],
    },
    {
      name: 'h1',
      title: 'H1 Text',
      type: 'string',
      fieldset: 'hero',
      validation: (Rule) => [Rule.required().error('H1 Text is required')],
    },
    {
      name: 'heroSubtitle',
      title: 'Hero Subtitle Text',
      type: 'heroBlockPT',
      fieldset: 'hero',
    },
    {
      name: 'heroImage',
      title: 'Hero Image',
      type: 'illustration',
      fieldset: 'hero',
      validation: (Rule) => [Rule.required().error('Field is required')],
    },
    {
      name: 'toc',
      title: 'Table of Contents',
      type: 'array',
      of: [{ type: 'tocLink' }],
      description: 'The order should match the in content appearance.',
      fieldset: 'mainContent',
    },
    {
      name: 'guideBody',
      type: 'guideBodyPT',
      title: 'Guide Content Body',
      fieldset: 'mainContent',
      validation: (Rule) => [Rule.required().error('Field is required')],
    },
  ],
};

{
      name: 'tileImage',
      type: 'illustration',
      validation: Rule =>
        Rule.custom(({ asset }) => {
          return asset ? true : 'An image is required';
        }),
    },