Hi Neanea,
You can use Paint (which comes included in any Windows PC) or use whichever image editor you prefer to use for creating or editing pictures for your own Little Math icon set.
Alternatively, you can use the Picture Editor tool that comes with Little Reader. To use Picture Editor outside of Little Reader, just go to Start > All Programs > BrillKids > Little Reader > Picture Editor.
For more details on how to use Picture Editor, please refer to this article:
http://support.brillkids.com/entries/20387493-how-to-use-the-picture-editorIn general, for Little Math icons, the idea is to isolate the object that you want to use to represent quantity. For example, if you have a picture of a car, you just need to remove or crop away anything else in the picture that you don't need (this can be other objects in the image, or blank/white spaces). As for the size of the image, Little Math will automatically resize your icon image to best fit your screen.
Once you're done creating or editing your images, the next step is to start adding the images to your Little Math icon set. For instructions on how to create icon sets, please refer to the "Icon Sets" section of this article:
http://support.brillkids.com/entries/20613661-how-to-create-and-edit-little-math-media-setsI hope this helps!