[SalesForce] files shared with a public link stored

You can programmatically create a public link from a ContentVersionId and then get the download URL like this:

ContentDistribution publicLink = new ContentDistribution(ContentVersionId = myContentVersionId, 
    Name='External Link',
    PreferencesNotifyOnVisit = false);

insert publicLink;

publicLink = [SELECT ContentDownloadUrl, ContentDocumentId
            FROM ContentDistribution 
            WHERE Id = :publicLink.Id];

System.debug('Download from: ' + publicLink.ContentDownloadUrl);

You can then use that as the src in an image.

Note that you need to be aware of limits if you're doing this:

the total number of content-delivery views allowed within a 24-hour period is limited to 20,000 and the amount of bandwidth allocated to content deliveries is limited to 20 GB. If recipients try to view deliveries after rate limit are exceeded, they’ll be prompted to try again later. Salesforce may be able to increase rate limits on an exception basis. For more information, contact your sales representative.

From https://help.salesforce.com/articleView?id=content_delivery_about.htm&type=5

So, if you're expecting high volume, you might still need to use a caching solution like CloudFront. See my old question here:

Using File/ContentDocument as an image in an unauthenticated community

If you're using these as content also sent from Marketing Cloud, you should be using Marketing Cloud Content Builder features including File Transfer Locations for Marketing Cloud Enhanced FTP Sites. Marketing Cloud tools will create the hyperlinks for you when you Create Content Blocks.

The source images, will not be accessible to external users if stored in the Account Object. Since you will be using the images for multiple purposes, I recommend you create a custom object to store them in. Create a custom hyperlink field in the Account Object if you want to display them on their related records. Let Journey Builder and other Marketing Tools do their parts separately when it comes to creating links for use with those particular tools. I don't think you're going to be able to create a single link from the Account to image that will serve for all your purposes.