Most image editors function in similar ways.
Hopefully, the following instructions can be applied to the image editor of your choice. Here's how to create an image using Corel. In CorelDraw, draw an empty rectangle close to the size of a graphic button ad or banner ad. To determine the size of the rectangle, partially export the empty object on a 1 to 1 ratio as a TIF file. During the export process, you should be able to read the actual size in pixels.