How to scale imagery precisely
0. Prepare your file to import an image object. Make sure you are prepared to manage a surface painted with your imagery once imported. Create layers or make components out of other geometry as needed.
1. Import your image as an image object. See File ->Import. Choose an image format and set the 2nd dropdown to "use as image". Select your file and click ok
2. Position and scale your image in your sketchup file to complete the import. Note that you'll likely want to position the image object away from other geometry because you'll need to explode it in the next step. Scale to some random size, you'll rescale it again later with the measure tool.
3. Right click on the image object and choose "Explode". This step is so you can turn the surface into a component which you can edit and scale inside the component context.
4. Make sure your selection is correct after exploding the image object. Then right click on your exploded image object geometry and make a component out of it. Now you're ready to scale based on a known distance in the image.
5. Edit the component. You can do this by double clicking with the select tool or right-clicking and choosing edit. Start the measure tool. Click option/alt to turn off drawing a guide line. Then measure the distance you know in the imagery. Next enter the value you want to scale that segment to. This is the distance you know. Hit enter and SU will ask you if you want to scale the current context. Click yes. Clean up the guide line if one was created. Then close the component edit session.
Matching the north arrow adjustments of Add Location imagery
Add Location rotates imagery and the north arrow attribute in the file. This is a legacy element to improve accuracy when importing multiple Add location grabs and then exporting to kmz. To see the north direct rotation value you'll need to download the north arrow extension - http://extensions.sketchup.com/en/content/solar-north
0. This section assumes you have already geo-located your model with Add Location. Measure the north direction rotation by going to The View Menu -> Tool palettes/Toolbars and make sure the "Solar North" toolbar is visible.
1. Click on the toggle north button to see which way the north direction has been rotated.
2. Click on "Enter north direction button" to see the exact rotation value to. At this point you can copy the value to your clipboard for later use in the rotate tool.
3. Select your newly scaled image object and use the rotate tool it to match your Add Location snapshot. Paste in the value from the solar north tool if you copied it.
4. At this point you should be able to move the imported image so that it matches up to your 2D Add Location snapshot.
Projecting textures onto terrain
If you want to project the imagery onto your 3D terrain check out this article for projecting textures - https://help.sketchup.com/en/article/3000114#project-texture