Here is the way I do it Don.
In the shot above you can see the rectangles (former sides of a box) set up for top, side and front views. Ignore the fact that there are two sets of rectangles as I used two different sets of 3-views to build my model in order to get all the details I required. In the above shot you can see I have selected the rectangle for the left view as it is shown as white.
Once the rectangle is selected you need to add a material to it and it needs to be a 'standard' G-Max material and
not a FSX material. Open the material editor, pick 'New', make it a 'Standard' material and then hit 'Apply'. This attaches a blank material to the rectangle and then you will be able to see any changes you make on the rectangle as you make them.
The material roll-out is shown below in more detail:
After you have applied the material to the rectangle go to the 'Maps' section and check the 'Diffuse Color' box and click the button beside that says 'None' (mine shows the name of the file as a texture is already applied).
This is the screen you should see now:
In the 'Bitmap Parameters' section click on the button next to 'Bitmap:' and navigate to the location of your side view.
If (as you describe) the image comes in pointing the wrong way then go up to the 'Coordinates' section and play with the values in the 'Angle' section. Depending on how you made your image you may have to adjust the 'U', 'V' or 'W' value by 90 or 180 degrees. Just adjust them one at a time until the image on the rectangle looks correct. NOTE: You can enter the numbers directly into the boxes rather than using the up/down spinners if you prefer.
Hope that helps.