The trick is to get the relationship id "rId" of the image. You get that by getting the TwoCellAnchor of the row/col in question. Then get the Embed. Use that to get the image.
[Test]
public void GetImageRelationshipIdAndImageOfThatId()
{
string row = "1";
string col = "0";
WorkbookPart wbPart = document.WorkbookPart;
var workSheet = wbPart.WorksheetParts.FirstOrDefault();
TwoCellAnchor cellHoldingPicture = workSheet.DrawingsPart.WorksheetDrawing.OfType<TwoCellAnchor>()
.Where(c => c.FromMarker.RowId.Text == row &&
c.FromMarker.ColumnId.Text == col).FirstOrDefault();
var picture = cellHoldingPicture.OfType<DocumentFormat.OpenXml.Drawing.Spreadsheet.Picture>().FirstOrDefault();
string rIdofPicture = picture.BlipFill.Blip.Embed;
Console.WriteLine("The rID of this Anchor's [{0},{1}] Picture is '{2}'" ,row,col, rIdofPicture);
ImagePart imageInThisCell = (ImagePart)workSheet.DrawingsPart.GetPartById(rIdofPicture);
}