On your Forms Server, browse to this path: C:\Program Files\Laserfiche\Laserfiche Forms\Forms\img

This is where you can store images that are most easily accessible from within Forms.

I recommend making a custom folder within the img folder just to keep things organized.

Then to access them you can just use a path like this: http://SERVERNAME/Forms/img/custom/large.jpg (this example is for a document named large.jpg, within a folder named custom, within the img folder mentioned above).

Then you can make this work easily with the Custom HTML element in Forms.  Here's a few examples:

1. Pre-sized images named small.jpg and large.jpg, clicking the small image opens the large image in a new window. 

   <a href="http://SERVERNAME/Forms/img/custom/large.jpg" target="_blank"><img src="http://SERVERNAME/Forms/img/custom/small.jpg"></a>

2. Pre-sized images named small.jpg and large.jpg, clicking the link below the small image opens the large image in a new window. 

   <img src="http://SERVERNAME/Forms/img/custom/small.jpg"><br><a href="http://SERVERNAME/Forms/img/custom/large.jpg" target="_blank">Click to See Larger Image</a>

   3. Just a large.jpg image, which is sized down in the form, and when it is clicked, it will display full size in a new window.

   <a href="http://SERVERNAME/Forms/img/custom/large.jpg" target="_blank"><img src="http://SERVERNAME/Forms/img/custom/large.jpg" width="100"></a>
   

You can rearrange any parts of those to meet your needs (example: on #2, move the <img> and <br> tags after the <a> tag to allow clicking on the image or the link.

You can also use relative paths instead of absolute paths for the images.
Instead of this: http://SERVERNAME/Forms/img/custom/large.jpg
You can do this: /Forms/img/custom/large.jpg