Whatever application you use, whether it is Gimp, Photoshop, or MSPaint, you need to plan out your idea ahead of time. Determined whether you need transparency or not. JPG files cannot handle transparency. PNG files cannot handle layers. Some applications do not even support either option for saving an image.
Whatever you decide to create, and how you do it, there are no extra steps when using an image as a texture for a heightfield or GUI icon. Using images as textures for objects, however, needs some attention. Try to always store your textures in the same folder hierarchy as when you were creating the 3D object that will use it. For TGEA, you will need to create materials out of the textures to properly view them.
The actual modeling of an object you wish to inject into Torque will vary between 3D modeling applications, such as 3D Studio Max or Maya. What should remain solid, however, is the process used to get a model from your application to the TGEA editor.
2. Install Exporter
3. Use Exporter utility to setup model for export
4. Export model
5. Copy .dts, .dsq, textures, etc. to your game/scriptsAndAssets/data/shapes/ directory. This is based on the gameExamples, so if you are storing them in your own unique location, use that folder instead.
6. Launch TGEA
7. Load a mission
8. Load the Mission Editor [F11]
9. Press [F3] to open the Mission Editor Creator
10. Locate where you placed your shape (usually in a filter called "Static Shapes"
11. Click on your object or drag it to the world
12. Use the Mission Editor Tools to manipulate your object
There are a few tools you can use to create DIFs. The most popular are Quark and Constructor. Both used brushes to create convex objects and portals that can make up buildings for you.
One of the differences to note is that Constructor is a tool developed by GarageGames, and was released free of charge for the GarageGames community. It is the best WYSIWYG DIF editor if you want the closest representation of what your object will look like in the game. It also has exporters built in for saving your scenes as .map, .dif, or .csx (constructor format).