Users can use their own custom markers for objects in STK. You can do this by opening the properties for your object and going to the 3D Graphics / Model properties page. Go to the section on the upper right side called Marker. If you check the Show box, the default selection is Marker, where you can select from a dot, cross, star, circle, X, box or triangle. The alternative is an Image File. The images that ship with STK are identical to the icons that appear in the GUI panel. However, you can make or use your own image files as markers in STK!
Some of the ones that I've made are shown below. This set is really just one marker in 8 different colors.
I’ve included the PSD (Photoshop) document for those of you who wish to edit these or make more in the attached zip file. Although the image files are 256 x 256 pixels, it is unlikely you will ever need that resolution.
After you point to the image file you wish to use in the GUI, always check the Transparent box so that you'll be able to see through them in your 3D window. For most situations you should limit your pixel size to between 15 and 30. For a slightly different look, check the Rotate box and set it to 45 degrees.
You will most likely need write permission to your C:\Program Files\AGI\STK 11\STKData\VO\Markers folder. That’s where STK stores all the 3D markers. You can actually put them anywhere you want, you’ll just have to remember where that is!
**It should also be noted that markers used in the 3D window are independent from markers used in the 2D window. That means that the markers you use in the 3D window may not always match the markers in the 2D window.**