How were you able to modify the Sidebar gadgets? Or did you build them from scratch?
The info gadgets on the rightmost monitor are Rainmeter gadgets, HUD skin. I'll update the description with that info.
The only sidebar gadgets are the Full Moon one (available on the MS site) and the slideshow gadgets that I did indeed modify. Assuming you mean the latter, here is some info...
Because they are MS delivered OS gadgets, you'll have to remove their security settings.
I made a copy of the SlideShow.Gadget directory, then renamed the current one to SlideShow.Gadget.old (make sure no slideshow gadgets are running).
Then copy the directory back and start tinkering with its files! The images are easy to edit in any common paint program. For example, I turned the images/on desktop/slideshow_glass_frame.png to an entirely clear, frameless version in Photoshop.
The main code is found in the en-US/js directory. I made a backup copy of slideShow.js as slideShow.js.old and started tinkering.
Some items are obvious like changing certain variables right at the beginning...
var maxUndockedWidth = 320; changed to 400
var maxUndockedHeight = 240; changed to 400
and, because I have a LOT of images in a lot of directories...
var gFolderCountMax = 500; changed to 10000
var gImageCountMax = 2000; changed to 500000
The rest were coding tweaks, made necessary due to the above changes breaking something, hehe...
gUndockFlag = true;
width=360, changed to 440
height=280; changed to 440
I did some other tweaks, like adding addition time length options, etc. but these are the major tweaks with made the slideshows larger and borderless.
In short, the idea is to backup the original data and then experiment a little. Swapping out the images is the easiest way to customize any Sidebar gadget, but you can also tweak the code if you want to.
I hope this helped.