Don't know much about NetObjects but this presumably has an html/source option, or just makes and saves the files, which you can access and edit before uploading. If you are not familiar with editing the source, or don't have an editor, open the page in IE on your computer and follow >File > Edit With Notepad. Or just right click the unopened file and do the same.
Editing the source code is nothing to be too concerned about, providing you are careful. At the top of the document, you will see an area starting with <head> and ending with </head> containing various lines, title, meta tags etc. Create a fresh line and enter the code below before </head>:
loadImage1 = new Image();
loadImage1.src = "images/second_image.jpg";
staticImage1 = new Image();
staticImage1.src = "images/first_image.jpg";
// End -->
Then, go down to the <body> section of the page and wherever the image is currently located, you will see a tag something like:
Probably more information before the /> but in any event, just delete the whole tag and replace this with:
<a href="#" onMouseOver="image1.src=loadImage1.src;" onMouseOut="image1.src=staticImage1.src;">
<img name="image1" src="images/first_image.jpg" border=0></a>
Really depends what you are trying to do but if this is just one simple image replacement on mouseover, the above should be fine. This isn't the only way to do what you want and for much more, not the best way but is the most straightforward. A few points:
You naturally need to change first_image.jpg and second_image.jpg to suit and may need to change the path i.e. if your images are not stored in a folder called images/
The href="#"part is in case you want the image that comes up to be a link, say to another page. If not, you can leave or remove this, won't matter. If you do want the image to be a link, chnage the tag to href="your_page.html" or whatever.
You can also add styles to an a tag, which you might want to, or need to include from the existing image e.g.
<a style="border: 3px solid red;" href="#" onMouseOver="image1.src=loadImage1.src;" onMouseOut="image1.src=staticImage1.src;">
<img name="image1" src="images/chateau_guests.jpg" border=0></a>
Where style="border: 3px solid red;" would make both images show with a 3px red border.
Hope there are no errors above, looks okay and really isn't as hard to implement as may seem the case. Find the folder containing your web pages, right click and copy a page to play with, then give this a try.