You can apply .grd files to Adobe Photoshop and use the colors in your picture than can be then saved as .jpg picture. It's actually not a file conversion, more of a workflow to process gradients and use them as colors. Another possible grd to jpg conversion unrelated to the previous one, would be an export of grid files in some of the GIS formats as plain JPEG pictures. This is a work for programs, such as the Total GIS Converter.
Open GRD file Open JPG fileA converter that is able to transform various GIS file formats