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Can I add one of my own images to the library ?
Not any way that I know of. However, you can use your image as a decal and render that. First, you need to put your image into the Texture folder. Go to your Application folder and right click on the TC app icon, click on "Show Package Contents", double click on "Contents", Resources, then "PhotoRender". Drag your image into the "Textures" folder. Close all the windows.
Launch TC and load your drawing. A decal needs to be placed on a surface so use the rectangle tool to make a rectangle the size you want the image to be. Select the "Cover" tool and make the rectangle a cover surface. Go to the "Render Library" -> "Decals" and put a decal in the center of your surface, any decal will work, we just want the decal data palette to work with. In the Inspector with the decal selected look in "Object Properties" -> "Data" tab. Set the decal using the height and width data boxes to make your decal cover the surface. Change "Masking" to "Factor", move the Factor slider to the right, Logo is set to "Image", and Image is set to your image that you put in the Texture folder. Click Apply and render it. Put your model in front of it and render again, hope this works for you.