I did exactly that. Though not sure if it is non-computer-nerd friendly, hahaha.
It's quite a time consuming process, especially when you do it for the first time.
I used a combination of software to get to the results as pleased.
I have used an add-on (plugin) in my Firefox browser called Fireshot to create a JPEG of a map. It is basically a print-screen function only you have the option to print screen the total webpage including the areas that are not visible in your browser window unless you scroll down/to the sides.
Then find the coordinates in google maps of the centre of the area you'd like to map. (open google maps and right click and choose 'What's here'). I've did this for you and the center of the island is about 9.734521,100.022278.
Initially I did it both in the Satellite view and the Terrain view. But now I only use the terrain view on my GPS system. However in Terrain view you can't zoom in as far as in Satellite view.
Copy below code (it includes instructions how to adapt), open Windows Notepad (if you are on MAC, I'm not sure what to use; perhaps just some html-editor).
Paste the code in notepad and save it. Now it will be saved as *.txt. Change the extension into *.html
Open it in notepad (or an html-editor) and adapt the iframe link following the instructions. You might have to save it every now and then and open the page in your internet browser to see if the area shows as you'd like.
Then click on the file to open it in your internet browser.
I use Firefox for which you need to download the above mentioned plugin called fireshot, there might be other options for other browsers.
You'll see the FireShot icon in your Firefox browser. Click the little arrow to open the menu and choose 'Capture Entire Page and ...'. After you click this you get a list of options. Choose 'save' and then save the file as JPG on your computer. You now have a jpg map of your area.
Here follows the code that you need to paste into your Windows Notepad. And adapt it according to the instructions.
<!--
Change t=...
EXAMPLE:
t=p is terrain view
t=h is satellite view
t=m is map view
Change ll=... , ...
EXAMPLE:
9.734521,100.022278
By right-clicking in a google maps it should show the option 'What's here'. It will show the coordinates of that location.
Choose the center of the area you'd like to map.
Change z=...
maximum zoom in terrain view is z=14; in satellite or map view you can zoom in even further.
Change width; in pixels
Change height; in pixels
Adapt the width and height untill the whole area you'd like to map is visible.
-->
<iframe width="1500" height="1700" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="https://maps.google.com/maps?num=1&t=p&ie=UTF8&z=14&ll=9.734521,100.022278&output=embed"></iframe>
Next step is to download Google Earth.
And follow the instructions on the following page:
garminbasecamp.wikispaces.com/Custom+Maps
If your total area jpeg is larger than 1024px x 1024px the Garmin system will reduce it, if you don't like that you need to split the image in multiple blocks of 1024 x 1024px and not more than 3MB.
If you upload too many custom maps to your system you can notice it gets significantly slower when starting up.
Not sure if this long post will scare you off, ;).
Anyway it might be useful for some, if not, I now created an online manual that I can use myself in the future when I forgot again how to do it ;).
Maybe somebody knows a better way.
And actually the first part to create the jpg map could be easy when the area is small, however I first tried it with Khao Yai and since it's huge it's a painful process...