Kentico doesn't store files in the web part or document type tables. It stores them in the CMS_Attachment table. You are correct, it's a foreign key GUID you're seeing.

One way to display the image is using the getattachment script. Here's a snippet from the documentation:

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Linking attachments

If you need to create a permanent link to a file uploaded to some document, such as DOC, PDF or image uploaded in the product form, you need to use a URL in the following format:

http://www.mydomain.com/getattachment/763c8921-be94-4610-99b4-25e8d3be5b08/logo.aspx

The URL consists of the following parts:

<domain>/getattachment/<file GUID>/<filename><extension>

The <file GUID> value is not the same as document GUID. It's a GUID of the file in the CMS_Attachment table. You can find this GUID if you display the image in the CMS Desk and see its URL.

The <file name> value can contain any text.

Hello,

The above post from Elijah is also correct and explains it very good.

I just want to add here that if you want to have a user friendly URL, you can take a look here - http://devnet.kentico.com/Knowledge-Base/API-and-Internals/How-to-display-link-to-a-file-or-document-without-.aspx.

It is mainly for document selector and for transformations, but the code is very similar and you can take inspiration from it. I hope it will help.

Best Regards,
Juraj Ondrus