Hi Emanuele,
In fact, what you are looking for is actually both simple and complicated, at the SAME time.
First, you need to specify WHAT is going to be used as a 'border', or 'frame', for your image. Of course, the list of possibilities is endless, infinite - so how do you propose a script or process that can handle that?
So, let's say that you simply want an 8-pixel wide border filled with a solid 'colour' of 50% 'gray'. Now, that becomes easy, because we just need a 'new image' that is 16 pixels taller and wider than the 'source' you are working with, and we just need to fill it with Rd=0.5, Gn=0.5 and Bu=0.5 - all quite trivially easy to do (so easy, in fact, that you really do NOT need a script, or process, to be able to achieve this).
Then, once you have your 'grey' image, you just use PixelMath to superimpose your 'source' onto the border (I won't trouble you with the actual expression just now - suffice to say that it is NOT complicated)
Finally, you want to add your 'signature' - by this, do you mean some rows of text (in which case the 'Signature' PJSR script might be all that you need), or do you mean a true Van Gogh 'squiggle' at the bottom right corner?
If you really need a 'squiggle', then it would again jsut be a case of 'scanning in' your authentic 'squiggle' and using PI to process this into a neat and tidy representation of your signature - and then, again, just using PixelMath to 'superimpose' this onto the image at the appropriate location.
At NO TIME would you have to work outside PixInsight - other than to get your signature 'scanned'.
And, knowledge of HOW TO DO THIS is actually very valuable PI knowledge in its own right - even more so if you actually solve the various stages yourself - learning just how powerful PixMath actually is.
Have a think - and if you struggle, I am sure that each step can be broken down into a demonstration, to encourage you to look at solving the next step yourself.
Who knows, you may even then write the final PJSR for your own use!!
Cheers,