There are two types of images you may consider added to the wiki - game images, and your own, copyright free, material.

Adding game images

Do not edit the image itself in any way. Do not remove whitespace, convert to other format or any of the sort.
Keep the image name as it is in the game if possible. If there is already an image with that name in the wiki, add a number add the end, staring with 1. So if, for example you are adding image.gif and there is already one in the wiki, call it image1.gif. If another image.gif is added it should be called image2.gif and so on.
Make a note of the image location. It should be something like "http://www.twilightheroes.com/images/<folder>/<imagename>". The location is case-sensitive.
As a description to the image use the {{THimage}} template. The template excepts two parameters, the first is <folder> and the second is <imagename>. Remember that both are case-sensitive.
If you changed the image name, add "renamed=1".
If everything you've done is correct the image will have a decoration with a link showing the original location. You can click it to see that everything was done correctly.
If not, edit the description.

Adding other material

Feel free to upload any other images, if you think they help the wiki community. However, don't upload any material which is copyrighted.
When adding such material it is advisable to state the source and purpose of the image in the description.
At the end of the description add "[[Category:User Contributed Images]]".