Hi Rakesh, 

As per my understanding, whenever you have an image saved within SFDC, this image file is hosted as a Salesforce file, but actually, it's converted into a servlet.
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And, what is it about? It means that whenever you're accessing the servlet URL, you're getting a preview within the SFDC environment, not accessing the file preview itself.
​
For accessing the image itself throughout an URL, you would need several steps to get a shareable link performing some Apex transformations, thereafter, you would be able to convert it into Base64.
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There's a standard nowadays, however, if Salesforce changes the way the URL is formed, the code will stop working.
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My recommendation is to host the images in a CDN (Content-Delivery-Network) such as Google Drive.
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But if you want to test, it follows the code I was able to get a shareable link (not needed auth, and you can reach out to the image file directly).
 

public with sharing class GeneratePublicLink {
    
   	//It must receive the objectId in which has a file attached.
    public static String createStructure(Id objectId){
        
        //Reference: https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_contentdocumentlink.htm
        ContentDocumentLink cdl = [ SELECT contentdocument.id
                                    FROM ContentDocumentLink 
                                    WHERE linkedentityid =: objectId 
                                    LIMIT 1];
        
        //Reference: https://developer.salesforce.com/docs/atlas.en-us.sfFieldRef.meta/sfFieldRef/salesforce_field_reference_ContentVersion.htm
        ContentVersion cVersion = [ SELECT Id, FileType, ContentBodyId
                                    FROM contentversion 
                                    WHERE contentdocumentid =: cdl.contentdocument.id ];
        
        //Reference: https://developer.salesforce.com/docs/atlas.en-us.sfFieldRef.meta/sfFieldRef/salesforce_field_reference_ContentDistribution.htm
        ContentDistribution cDistribuition = new ContentDistribution();
        cDistribuition.Name = 'Test';
        cDistribuition.ContentVersionId = cVersion.id;
        cDistribuition.PreferencesAllowViewInBrowser= true;
        cDistribuition.PreferencesLinkLatestVersion=true;
        cDistribuition.PreferencesNotifyOnVisit=false;
        cDistribuition.PreferencesPasswordRequired=false;
        cDistribuition.PreferencesAllowOriginalDownload= true;
        
        insert cDistribuition;
        
        ContentDistribution cdCreated = [ SELECT Id, DistributionPublicUrl 
                                          FROM ContentDistribution
                                          WHERE Id =: cDistribuition.Id ];
        
        return createPublicLink(cVersion, cdCreated);        
    }
    
    private static String createPublicLink(ContentVersion cVersion, ContentDistribution cDist) {
        
        String fileType = cVersion.FileType.toLowerCase().capitalize();
        String cVersionId = cVersion.Id;
        String cBodyId = cVersion.ContentBodyId;
        String originalUrl = String.valueOf(cDist.DistributionPublicUrl);
        String cDistParam = originalUrl.substring(originalUrl.indexOf('/a/'));
        String orgId = ConnectApi.Organization.getSettings().orgId;
        String baseDomain = Url.getOrgDomainUrl().toExternalForm();
        baseDomain = baseDomain.substringBefore('.') + '--c.documentforce.com';
        
        String theUrl = baseDomain + '/sfc/dist/version/renditionDownload?rendition=ORIGINAL_' +
            fileType + '&versionId=' + cVersionId + '&operationContext=DELIVERY&contentId=' + cBodyId +
            '&page=0&d=' + cDistParam + '&oid=' + orgId + '&dpt=null&viewId=';
        
        return theUrl;
    }
}

Let me know if this helps, if it does, please mark this answer as best so that others facing the same issue will find this information useful. Thank you