I suggest you ignore the non-constructive post (Untrue) above.
I also suggest:
1: Get GIMP 2 (or a layer-supporting image editor)
2: Get
http://yrarg.cncguild.net/index.php?page=tutorials/cameomk2 the image there
3: Using "screen" layer mode, combine your cameo with the PNG in the link's zip file.
4: Dont use color index 0.
In addition:
Edit the image. Crop it if you feel the unit is too small relative to the image size. If it comes out ugly in the RA2 cameo palette, edit the colors. Change the color of the sky, ground, unit individually.
Tweaking contrast and brightness is also a good idea.
Also, adding the RA2 cameo palette to your image editor can be very helpful in speeding up the tweaking process.
On to the critique:
Of course, this is all dependent on your image choice. I scrolled through 14 (!) pages of Google Image results for Challenger II and had to switch to google.co.uk in order to find the image you used.
-Challenger II: Angle and color make this unusable without serious overhaul. There are tons of MUCH better challenger images.
-Apocalypse Tank: This cameo has been done already, and done better.
-Sniper: You can't even tell what's going on here.
-Missing Cameo- Washed out and zoomed out
-Apocalypse Tank 2 (missing cameo): The Apocalypse Tank is missing too. You can only see half of the vehicle.
-Psy Dom: The best of the bunch. Could be a bit smoother.
-Howitzer: Needs to be zoomed in more, smoother. Contrast could be higher.
-Apache: Definitely needs to be smoother. Otherwise it's a pretty interesting cameo.
In other words, you need to use better images.
edit- and cut it with the polls.