I can't speak to whether or not the upload can be changed, and I assume you are self-hosted and not on Cloud.  Here's how I do it for forms in my self-hosted environment.

I usually just put the image files on the server(s) and then use HTML img tags to display them.

Let's say I have an image named picture.png that I need to display on my form.  I need it to work both internally if my staff are accessing the Form via authenticating into the internal server, and externally if public users are accessing the Form that way, so I'll put the image on both servers in the same location.

I go to C:\Program Files\Laserfiche\Laserfiche Forms\Forms\img on the servers, and then add a subfolder called custom or something like that, and put my image file in there.  Example:
C:\Program Files\Laserfiche\Laserfiche Forms\Forms\img\custom\picture.png
Again - this is on BOTH the internal and external servers.

Then on my form, in the Custom HTML element, I add an img tag like this: 

<img src="/forms/img/custom/picture.png">

If you haven't changed the default set-up, then the base URL for Forms is pointing to the C:\Program Files\Laserfiche\Laserfiche Forms folder, so pointing to /forms/img/custom/picture.png will direct to C:\Program Files\Laserfiche\Laserfiche Forms\Forms\img\custom\picture.png.  Because I set-up the same name and location of the image file on both servers, it works regardless of which server is used to access the form.

Does that all make sense?