Opening the image file or using it to create a grid-based map (e.g. an image map or color relief map) will convert the pixel color to a Z value ranging from 0 to 1. This is only recommended if the image file in question actually contains Z values for the pixels. In most cases, you would import imagery as base maps.
For background information, the File | Open command opens the image in the Grid Editor. In Surfer 13 and earlier versions, the Grid Editor always displays information in black and white. Each grid node (pixels in this case) is represented by a small cross (+). You can select each grid node and the X, Y and Z values of that grid node are reported at the top of the window. The X and Y coordinates are the pixel coordinates (unless it is georeferenced) and the Z values are calculated from the color of the image (unless it is a particular type of image file that contains a Z value for each pixel) and will range from 0 to 1. You can use the Grid Editor to edit the Z values of any selected grid node.
Since Surfer allows you to use a TIF file as a grid file, you can also use the image to create a color relief map (or any other grid-based map). The default color scale for a color relief map is Terrain (what you see on the screen). You can select the Color Relief layer in the Contents window, and in the Properties window click to the right of Colors, and change the color scale to anything you want. However, an image map is not the same as loading an image as a base map.
Updated October 4, 2017