Inkodye is a light-reactive dye made by the Lumi company. There are a lot of good instructables for making stuff with Inkodye - this instructable will show you how modify an image to be printed on a transparency and used as a negative for the Inkodye process.
The software we're going to use for this is called the GNU Image Manipulation Program - GIMP for short. GIMP is Free and Open Source Software, which means you can download and use it for whatever you'd like at no cost :)
Despite being free to use, GIMP is very powerful software and is often compared Adobe Photoshop. Because of its many features, it can be overwhelming, but in this Instructable, I'll show you only what you need to know to create a transparency negative for Inkodye.
You can download GIMP from the gimp.org website. Make sure you download directly from gimp.org - some nefarious people repackage GIMP with malware, but the downloads on gimp.org are safe!
Once you have it installed, proceed to the next step!
Note: If you're interested in the more advanced features of GIMP, there are plenty of resources available around the Internet!
Step 1: Prepare GIMP for First Use and Open Your File
If this is your first time using GIMP, you'll probably want to change the view. By default, GIMP separates the workspace, tools, and other dialogs. I find it easier to use if everything is one window.
To set this, open the Window menu and select Single Window Mode.
Next, open your image. You can do this by opening the File menu and selecting Open. You can also use CTRL+O.
For this Instructable, I'll be using Tyrannosaurus Rey.jpg by Luis Rey, which I found on Wikimedia.
Step 2: Remove the Color
Now we need to remove the color from our image. The Inkodye process works by allowing light in some places and blocking light in others. Having color on our transparency makes no difference, so we'll remove the color to make a better visual of the contrast. It's also cheaper to print!
To remove the color from your image, click on the Colors menu and select Hue-Saturation.
With the Hue-Saturation dialog open, find the Saturation slider, and drag it all the way to the left. This will remove all of the color, making your image grayscale.
Click the OK button to close the dialog.
Note: You can adjust the brightness and contrast settings by opening the Colors menu again and selecting Brightness-Contrast.
Step 3: Create the Negative
The Inkodye process requires a negative. In other words, we need to block the light where we don't want color to show, so these areas will need to be black. Wherever we do want color, we'll should be white.
In my print, I want dark parts like the T-Rex to receive the color, and the white parts, such as the sky, to remain un-dyed.
To accomplish this, we can invert the colors. Open the Colors menu again and select Invert.
Step 4: Set the Print Size
The next step is to set the physical size of the our image. Right now, our image is measured in "pixels", which don't directly correlate to real-world dimensions. You'll need to decide what physical size you want your printed image to be and configure the software with those parameters.
When you know what size you want, click on the Image menu and select Print Size.
Just to the right of the Width and Height settings is a dropdown box for adjusting the unit. Set this to whatever units you're comfortable with before you start changing the numbers.
Click the OK button to close the dialog.
Step 5: Export the PDF
If you're taking your image to a print shop, you'll probably want to bring them a PDF file to ensure they print the correct size.
To do this, open the File menu and select Export.
In the dialog, be sure to save with the file extension .PDF. You can also set the file type by using the "Select File Type (By Extension)" option at the bottom of the dialog. You can save the PDF to a flashdrive and take it to a print shop to be printed, or print it yourself directly from GIMP.
Once you have your transparency, you're ready to continue the Inkodye process! Have fun!