For security reasons, uploading images willy nilly in admin is a bad idea. One workaround is extremely helpful after you get used to it. The instructions are at http://www.teamwiztech.com/blog/how-to-upload-extra-images-for-zen-cart/.
You can also use image handler, if installed to upload under tools, but the advantage of uploading images without image handler is that you can name the images the way you want which can help SEO by repeating your keywords. You can also use an FTP client to upload directly to the server but most folks don't want to fool with that, and it is not for the timid to learn. You have to have software (which is free like Filezilla) and figure it out. So that's actually a steeper learning curve. Also, it can be dangerous if you get too brave and upload something to the wrong place!
After uploading the image, you click on the image button in the editor - looks like a square with a mountain or a tree in it. You then type in the path to the image but do it this way: /images/image.name - putting the / before the images will enable you to see the image in admin. This isn't quite as easy as it could be but it the best way for to work with images in Zen Cart category/product descriptions or define pages and EZ pages.