How to get full-size image from image field in PowerApps using IOrganizationService?

Question

In PowerApps, let's say I have a field of "image" type called foo_imagefield on a table foo_testtable. I want to access the full size of this image in a plugin or console app using IOrganizationService. I can retrieve the record with the image field as follows:

string[] columns = { "foo_imagefield" };
Entity testRecord = Service.Retrieve("foo_testtable", new Guid("4B365AFD-B31C-EC11-B6E6-000D3A4EA781"), new ColumnSet(columns));

Now I can get the image bytes from the field:

byte[] imageBytes = testRecord.GetAttributeValue<byte[]>("foo_imagefield");

Great, except this only seems to give me the thumbnail version of the image (I believe PowerApps reduces it to 144px * 144px). It's super tiny. I know I can get the full version via a URL (e.g. https://myinstance.crm.dynamics.com/Image/download.aspx?Entity=foo_testtable&Attribute=foo_imagefield&Id=4b365afd-b31c-ec11-b6e6-000d3a4ea781&Timestamp=637801267356898020&Full=true as long as you are authenticated). I can also get the full version via rest API (e.g. GET /api/data/v9.1/<entity-type(id)>/<image-attribute-name>/$value?size=full). But being that I'm doing this in a plugin, I don't want to have to authenticate again.

Is there any way to get the full image with Microsoft.CRM.SDK IOrganizationService?