First, wrong forum. See the other Support forum.
Second, you're confusing the "Tools>Options" transparent color setting with actually picking the transparent color when importing a battler graphic.
When importing battlers (that's what you meant by monsters, correct?), if you don't use PNG, you won't be able to set a transparent color. Use the Resource Manager to add your monsters by clicking Import.. and picking the graphic. Then you left-click on a spot with white color to set the transparency. That's all you should need to do. I'll add here that, most of the time, it will look ugly and blotchy because white is a common color that shows up in battler graphics.
You can't change that option you are talking about to total white - it goes to pure transparent, which is grey-white boxes, I believe, in this program. It has no effect on your graphics except changing the color you see when looking at the parts of an image that are transparent. In other words, it's cosmetic.
If you have any more questions, please use the proper forum next time