The problem is with plugins that don't fade to nothing by the edge of the quad. This creates an obvious 'light box' around the plugin which looks really bad.
Check out this example
The image looks fine. It should make a lovely plugin yes?
This is what it looks like in AlamDV.
Not very good
With luminosity effects this is caused by the image not being completely black by the time it gets to the edge. Even a couple of shades lighter than black can look really obvious when in AlamDV.
The way to fix this is to simply make sure you don't crop the image till its disappeared! Don't do this by eye, 9 times out of 10 you'll get it wrong. Most paint programs have tools that let you create a flood selection (in psp its called the magic wand). Simply select all the pure black around the edge of the plugin. Then invert the selection and you will end up with the bits of the image that you must not crop (ie this is the smallest size your image must be)
Obviously if a submitted plugin has been cropped too much I have no way to uncrop it. So what I do is simply darken the edges of the image until they are properly black. This method isn't ideal and it would be much better if the submitter had just cropped it right in the first place.
The same is true with alpha plugins. The alpha channel should make the image completly transparent by the time it gets to the edge of the quad.