SC2Mapster

(Sorry, I couldn't figure out adding pictures) I hope I explain things clearly enough.
First of all, you will need GIMP and the .dds plugin. They are both entirely free and trustable tools. I will be making a icon of a Warhound. It sorta makes sense because the current Warhound icon is not up to date with the model.

Step One: Lighting and background
               For a mech, I suggest you use grass as a background. and for lighting you should a doodad floodlight(A tall one more preferably). Make sure the unit is in the direct light.

Step Two: View and Screenshot
               Zoom in on the unit at any angle or any zoom level, when you find a angle and zoom level you like, click "prt sc sus rp" second above the delete
               key, it will create a screenshot of the editors view. The screenshot will likely be located in Documents/StarCraft II/Screenshots.

Step Three: Image Sizing and Configuration
               Load the image in GIMP. The screenshot dimensions are 1589x910. Try setting the image canvas size(In the Image tab) to 910x910. There will  likely be a need to offset the image centering so that all of the unit in the image is still in the picture. Then set layer to image size(In layer tab). After that, you to SCALE the picture(In Image
               tab) to 76x76.

Step Four: Image background
               Once your image is 76x76, zoom in at 800%. Replace all the background grass and anything besides the unit with the color black. As soon as this
               is done, select the outside 4 pixels on each side and use the "Color to Alpha" in the color tab to your selection, make sure that the color select in     
               color to alpha is black. Everything in the selection will be transparent(Vied as checkered light and dark grey). Make sure to unselect.

Step Five: Finishing Touches
               Select the blur tool in the tool palette and blur the edges of the black all around, after you have blurred to your liking, get the tool options and
               select sharpen, same button on tool palette as blur. Make the brush larger than the unit in the image and center it and click three times with
               opacity at 100. After that unselect everything, if you do. Zoom out and click "Export As" in the file tab, name it, and make sure you save as .dds.
               Your icon should now be ready to import and be used.

Edit: As you can probably tell, sometimes I do better than other times, I did not do my best with Warhound icon.