When adding a logo or graphic overlay in the 2D or 3D graphics window (not to be confused with chunk image inlays on the globe), STK uses the uppermost left pixel of the image as the transparent color. To define a transparent background, make the upper left pixel color the same as the logo's background color. If you do not want transparency, then choose a color for the upper left pixel that does not match any other color in the image.
For example, if the upper left pixel of the image is white (RGB 255,255,255), then STK treats every white pixel as the transparent color.
Use a program such as Photoshop, Paint Shop Pro, etc., to get a histogram of colors for the image and to change the value of the upper left pixel.
To learn more about using overlays in the 2D or 3D graphics window, including all the file formats STK accepts for graphic overlays, refer to the STK Help.
For overlays on the 2D windowFor overlays on the 3D window